What's the best opening for an beginner ?

For someone seeking help with choosing openings, I usually bring up Openings for Amateurs by Pete Tamburro (2014).
http://kenilworthian.blogspot.com/2014/05/review-of-pete-tamburros-openings-for.html
https://chessbookreviews.wordpress.com/tag/openings-for-amateurs/
I believe that it is possible to see a fair portion of the beginning of Tamburro's book by going to the Mongoose Press site.
https://www.mongoosepress.com/catalog/excerpts/openings_amateurs.pdf
Perhaps Thaddeus111 would also want to look at Discovering Chess Openings by GM John Emms (2006).
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf
"There is no such thing as a 'best opening.' Each player should choose an opening that attracts him. Some players are looking for a gambit as White, others for Black gambits. Many players that are starting out (or have bad memories) want to avoid mainstream systems, others want dynamic openings, and others want calm positional pathways. It’s all about personal taste and personal need.
For example, if you feel you’re poor at tactics you can choose a quiet positional opening (trying to hide from your weakness and just play chess), or seek more dynamic openings that engender lots of tactics and sacrifices (this might lead to more losses but, over time, will improve your tactical skills and make you stronger)." - IM Jeremy Silman (January 28, 2016)
https://www.chess.com/article/view/opening-questions-and-a-dream-mate
https://www.chess.com/article/view/picking-the-correct-opening-repertoire
http://chess-teacher.com/best-chess-openings/
https://www.chess.com/blog/TigerLilov/build-your-opening-repertoire
https://www.chess.com/blog/CraiggoryC/how-to-build-an-opening-repertoire
https://www.chess.com/article/view/learning-an-opening-to-memorize-or-understand
https://www.chess.com/article/view/the-perfect-opening-for-the-lazy-student
https://www.chess.com/article/view/3-ways-to-learn-new-openings
https://www.chess.com/article/view/how-to-understand-openings
https://www.newinchess.com/media/wysiwyg/product_pdf/9035.pdf
https://web.archive.org/web/20140627110453/http://www.chesscafe.com/text/hansen169.pdf
https://www.newinchess.com/media/wysiwyg/product_pdf/9029.pdf
https://www.newinchess.com/media/wysiwyg/product_pdf/7277.pdf
https://www.newinchess.com/media/wysiwyg/product_pdf/9033.pdf
https://www.newinchess.com/media/wysiwyg/product_pdf/9050.pdf
https://web.archive.org/web/20140627104938/http://www.chesscafe.com/text/hansen159.pdf
https://web.archive.org/web/20140627022042/http://www.chesscafe.com/text/hansen153.pdf
https://web.archive.org/web/20140627132508/http://www.chesscafe.com/text/hansen173.pdf
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... Once you identify an opening you really like and wish to learn in more depth, then should you pick up a book on a particular opening or variation. Start with ones that explain the opening variations and are not just meant for advanced players. ..." - Dan Heisman (2001)
https://web.archive.org/web/20140626180930/http://www.chesscafe.com/text/heisman06.pdf
"... To begin with, only study the main lines ... you can easily fill in the unusual lines later. ..." - GM John Nunn (2006)
"... I feel that the main reasons to buy an opening book are to give a good overview of the opening, and to explain general plans and ideas. ..." - GM John Nunn (2006)
"... If the book contains illustrative games, it is worth playing these over first ..." - GM John Nunn (2006)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"... For inexperienced players, I think the model that bases opening discussions on more or less complete games that are fully annotated, though with a main focus on the opening and early middlegame, is the ideal. ..." - FM Carsten Hansen (2010)
"... Everyman Chess has started a new series aimed at those who want to understand the basics of an opening, i.e., the not-yet-so-strong players. ... I imagine [there] will be a long series based on the premise of bringing the basic ideas of an opening to the reader through plenty of introductory text, game annotations, hints, plans and much more. ..." - FM Carsten Hansen (2002)
https://web.archive.org/web/20140627055734/http://www.chesscafe.com/text/hansen38.pdf
"The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line." - GM Nigel Davies (2005)

"OPEN games and only study endgames" -boring orthodox chess lame-os who don't care about the beauty of the game, SON.

I think it has a lot to do with tactics and calculation. You could know everything about the Najdorf but if you miscalculate or drop pieces it's practically useless knowledge. A lot of endgames are pretty much "do this or you' re screwed." Open games can be like that too. But that's just my theory.

"... In the middlegame and especially the endgame you can get a long way through relying on general principles and the calculation of variations; in the opening you can go very wrong very quickly if you don't know what ideas have worked and what haven't in the past. It has taken hundreds of years of trial and error by great minds like Alekhine and, in our day, Kasparov to reach our current knowledge of the openings. ..." - GM Neil McDonald (2001)

I highly recommend to look into two opening repertoire books by Vincent Moret.  The books provide a complete opening repertoire targeted to beginner-intermediate players.  The repertoires focus mainly on aggressive but sound lines for which understanding the typical plans and strategies are more important than memorization of myriad lines and variations....

My First Chess Opening Repertoire for White....

https://www.amazon.com/First-Chess-Opening-Repertoire-White/dp/9056916335/ref=sr_1_3?s=books&ie=UTF8&qid=1515364956&sr=1-3&keywords=vincent+more

My First Chess Opening Repertoire for Black...

https://www.amazon.com/First-Chess-Opening-Repertoire-Ready/dp/9056917463/ref=sr_1_1?s=books&ie=UTF8&qid=1515364956&sr=1-1&keywords=vincent+moret

Introduction to Moret's White repertoire...

https://www.newinchess.com/media/wysiwyg/product_pdf/9033.pdf

More on Moret's White repertoire...

https://www.youtube.com/watch?v=f41ZbPq9OpE

https://www.chessable.com/opening-book/my-first-chess-opening-repertoire/7543/

See my mini review of Moret's White opening repertoire book here (search Moret)....

Good Chess Openings Books for Beginners and Beyond...

https://www.chess.com/blog/RussBell/good-chess-openings-books-for-beginners-and-beyond

Introduction to Moret's Black Repertoire...

https://www.chess.com/blog/IndreRe/book-review-vincent-moret-my-first-chess-opening-repertoire-for-black

https://www.newinchess.com/media/wysiwyg/product_pdf/9050.pdf

Here are some opening repertoire suggestions by IM Andrew Martin and GM Nigel Davies....click on their respective names at the top of the web page...

http://www.chesspublishing.com/content/repert.htm

Finally, the following articles relating to choosing an opening repertoire offer food for thought...

http://www.mark-weeks.com/aboutcom/aa02i07.htm

A three-part set of articles by GrandPatzerChess - Parts 1 & 2 are concerned with his thoughts on building a White repertoire.  Part 3 with a Black repertoire.  I found the articles on the White repertoire to be somewhat confusing and even contradictory in places.  Part 3 on the Black repertoire seems to be a little more coherent.  Nevertheless I found the articles to be generally interesting and thought provoking...

http://grandpatzerchess.blogspot.com/2007/03/openings-for-improving-players-part-1.html

http://grandpatzerchess.blogspot.com/2007/03/openings-for-improving-players-part-2.html

http://grandpatzerchess.blogspot.com/2007/03/openings-for-improving-players-part-3.html

LOnDON SYSTEM and ruy LOPEZ are the best

One company that hired me as a chess tutor instructed us to teach the Italian Game since it's the most logical for beginners. I'd agree with that, especially since it avoids the extensive theory of the Ruy Lopez and Sicilian, the excess defensiveness of the French Defense and Philidor's Defense, the intricacies of 1. d4, and the tactics of gambits like the King's Gambit or Queen's Gambit. Beginners should also avoid any hypermodern openings and systems like the Colle System (although appealing, they are not recommended for some reason, maybe because they aren't responsive enough to what the opponent is doing and therefore teach bad habits), so about the only good opening not excluded from all those guidelines is the Italian Game.

Note the Vincent Moret's "My First Chess Opening Repertoire for White" features the Italian Game against 1.e4 e5.

The following also deal exclusively with the Italian game: "Beating 1 e4 e5: A Repertoire for White in the Open Games" and  "Winning With the Slow (but Venomous!) Italian" by Muller & Souleidis.

Some instructional DVDs on the Italian Game recommended here:

http://chessimprover.com/coming-full-circle-to-the-giuoco-piano/

Possibly helpful:

Starting Out: Open Games by GM Glenn Flear (2010)
https://web.archive.org/web/20140626232452/http://www.chesscafe.com/text/hansen134.pdf

My First Chess Opening Repertoire for White by Vincent Moret
https://www.newinchess.com/media/wysiwyg/product_pdf/9033.pdf
https://www.chess.com/article/view/how-to-understand-openings
A SIMPLE CHESS OPENING REPERTOIRE FOR WHITE by Sam Collins
http://www.gambitbooks.com/pdfs/A_Simple_Chess_Opening_Repertoire_for_White.pdf
http://www.jeremysilman.com/shop/pc/A-Simple-Chess-Opening-Repertoire-for-White-76p3916.htm
Winning with the Slow (but Venomous!) Italian by Karsten Müller and Georgios Souleidis
https://www.newinchess.com/media/wysiwyg/product_pdf/9037.pdf

The beginner should look at all the openings and decide which positions look the most comfortable. 

This opening.

 That's the Four Knights Game, Scotch Variation.

Many beginners think their game is poor because they do not know openings. The conviction increases because after learning some openings they can withstand some extra moves, for this reason they think that getting to know them well could lead them to draw against strong players. But that is not, once the opening is over they will lose in some move and do not realize that all the time they have lost to learning so well opening is time taken to improve the game at all its stages.

for a beginner, start with e4

This.

It's most logical to start with the open games, play 1 e4 for White & 1...e5 for Black, & answer 1 d4 with ...d5. These are the basics.

Also, tactics & endings are far more important. You lose most of your games to one or the other, not because you played the opening poorly.

I'm definitely a beginner myself and I find the openings I do best with are:

As White:

I always start e4 then play accordingly based on Black's response.

Lots of low rated players seem to commonly reply with the Phildor, Scandanavian, Nimzowitsch, and occasionally the Sicilian. So learn at least the basic ideas of those openings.

As Black:

Against e4 I prefer the Sicilian by FAR over any opening. 

Against d4 I prefer Nf6 and going into whatever Indian game follows.

Against anything else I either play e5 or d5 depending on their move and see what transpires.