Best chess opening? Both black and white...

King's Pawn game 1. e4 e5.

KPG is probably the best for any beginner.

Around 2010, IM John Watson wrote, "... For players with very limited experience, ... the Sicilian Defence ... normally leaves you with little room to manoeuvre and is best left until your positional skills develop. ... I'm still not excited about my students playing the Sicilian Defence at [the stage where they have a moderate level of experience and some opening competence], because it almost always means playing with less space and development, and in some cases with exotic and not particularly instructive pawn-structures. ... if you're taking the Sicilian up at [say, 1700 Elo and above], you should put in a lot of serious study time, as well as commit to playing it for a few years. ..."

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 Olof02 would also want to consider Discovering Chess Openings by GM John Emms (2006).
"... For beginning players, [Discovering Chess Openings] will offer an opportunity to start out on the right foot and really get a feel for what is happening on the board. ..." - FM Carsten Hansen (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/3-ways-to-learn-new-openings
https://www.chess.com/article/view/how-to-understand-openings
https://www.chess.com/article/view/the-perfect-opening-for-the-lazy-student
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.chess.com/article/view/has-the-king-s-indian-attack-been-forgotten
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)

English opening for white (1. c4).

Owen's/English Defense for black (1... b6).

Fly-Eagle-Fly: What the heck ?!? Coca-Cola gambit  That's the truly patriotic American opening. 

https://www.chess.com/openings/A00-Grob-Opening-Double-Grob-Coca-Cola-Gambit

A BEGINNER should not be playing flank openings.

The first two openings any beginner should study is the Ruy Lopez and Queen's Gambit and PLAY BOTH FROM BOTH SIDES!

Play simple things, like modern defense, and meises. Pretty much the same every time.

BTW to what ply?

"Alekhine advised beginners not to play the Spanish game. We also recommend you get some experience first by playing relatively simple openings - the Scotch and Italian games - and only then move on to the Spanish one." - Journey to the Chess Kingdom by Yuri Averbakh and Mikhail Beilin

Best advice is read and apply this. 

https://www.chess.com/article/view/study-plan-for-beginners-the-opening2

it suggests starting with 

• The Ruy Lopez or Spanish Game
• The Giuoco Piano or Italian Game
• The Four Knights Opening
• The Two Knights or "Fried Liver"
• The Queen's Gambit Accepted
• The Queen's Gambit Declined
• The Queen's Gambit Declined: Slav and/or Semi-Slav

however, before getting into this be sure you know the principles of the opening

"... 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)

"Generally speaking, 'Starting Out' and 'Sicilian Najdorf' are not exactly words that one envisions in the same title, because anyone who is just starting out should not dive into the vast ocean of theory that is the Najdorf. For beginners, the time invested in studying even minor lines can be more productively used solving tactical puzzles and basic endgame technique.

...

... In some lines, a good understanding of basic principles will take you far, while in others, such as the Poisoned Pawn (6 Bg5 e6 7 f4 Qb6!?), memorization is a must, as one wrong move can cost you the game in the blink of an eye. ..." - FM Carsten Hansen (2006)

https://web.archive.org/web/20140626175558/http://www.chesscafe.com/text/hansen87.pdf

Phoenyx75 wrote: "... what do you do if White plays something other than 2.Nf3 after 1.e4? ..."

Possibly of interest:

Starting Out: The Sicilian, 2nd Edition

https://web.archive.org/web/20140627122350/http://www.chesscafe.com/text/hansen123.pdf

Anti-Sicilians: Move by Move

https://www.everymanchess.com/downloadable/download/sample/sample_id/119/

Sveshnikov vs. the Anti-Sicilians

https://www.newinchess.com/media/wysiwyg/product_pdf/9003.pdf

http://www.jeremysilman.com/shop/pc/Sveshnikov-vs-The-Anti-Sicilians-76p3890.htm

Grandmaster Repertoire 6A - Beating the Anti-Sicilians by Vassilios Kotronias

http://www.qualitychess.co.uk/ebooks/BeatingtheAnti-Sicilians-excerpt.pdf