Is the QGD objectively best against 1.d4

Maybe for patzers it's all the same but it still seems like a good response to 1.d4

It could be "the best" but only subjectively.

No I mean it's universally played almost exclusively at the top levels so it has got to be the best.

Okay, U can compare stats in the Women World Championship (still in progress):

https://www.chess.com/forum/view/chess-openings/womens-world-championship-ko-2018-openings

What is your criteria in order for an opening or defence to be considered "objectively best"?

criteria for objectively best would be that it is theoretically best i.e. employed by all the strongest engines and Grandmasters.

objectively best means independent of preference or style. My coach likes to play the Grunfeld and he doesn't know anything else against 1.d4. I wouldn't call that an objective choice. It's dependent on his style.

The engines say it's the best but I prefer the Austrian defense

player should play the opening, not based on statistics, but the one in which he feels most comfortable at moment. QGD is good choice when playing against much strong guys when solid structure may provide good chances for draw, even win if opponent apply to much risk. but if I played against lower rated players, they may set up position and do nothing, then it may be very hard to score, in these cases I rather chose KID, grunfeld or whatever with Bg7 .

If you play QGD against a stronger player he will know the opening more than you becouse it's the most common so that's no good advice

TINSTAABO

"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
The November 2018 issue of Chess lists the top twenty openings compiled from a list of 2345 September games where both players were rated over 2400 Elo. One can not take position on this list too seriously because it is greatly influenced by how the openings are grouped. For example, all the Retis are grouped together, while English is separated into 1...c5, 1...e5, etc. Nevertheless, for what it is worth, some of the list entries are: 158 Retis, 127 King's Indians, 91 declined Queen's Gambits, 88 Caro-Kanns, 70 Najdorf Sicilians, 64 Slavs, 63 Nimzo-Indians, 53 1...Nf6 Englishes, 48 Kan Sicilians, 48 Catalans, 42 Berlin Ruy Lopezes, 42 Queen's Indians, 38 1...e6 Englishes, 37 3 Nxe5 Petroffes, 36 1...e5 Englishes, and 33 Closed Ruy Lopezes.

"... A typical way of choosing an opening repertoire is to copy the openings used by a player one admires. ... However, what is good at world-championship level is not always the best choice at lower levels of play, and it is often a good idea to choose a 'model' who is nearer your own playing strength. ..." - FM Steve Giddins (2008)

my own reason to play the QGD is that I like the pawn structure.

There are several objectively good responses to d4...

advantage of QGD is that is very solid, and actually there is no need to know too much theory, even if after lets say move 10.  black does not know any more theory, still plans and ideas are very clear, and generally position itself.

Do U mean the Semi-Slav V-structure for black?

In chess,com Openings Explorer's Master Games, there were:

68,329 2...c6, the Slav Defense

52,819 2...e6, the Queen's Gambit Declined

17,870 2...dxc4, the Queen's Gambit Accepted

~6599 2...other responses.

So the Queen's Gambit Declined is NOT "almost exclusively played at top levels" and isn't even the top choice among masters.  If by "top" you mean the few at the very top, note that Carlsen, Anand, Ivanchuk and Shirov, among many others, often play the Slav either normally or by transposition, and it has been popular for over a century: loved by World Champs Euwe, Botvinnik, Smyslov, and others.

Personally, I am not positionally as skilled as master players and the Queen's Gambit Declined means my Queen's Bishop stays out of action for too long for me after ...e6 blocks it in as a true "Bad Bishop."

But the Slav lets me get my Queen's Bishop to f5 or g4 before my Pawn Chain slams the door on it. As Jeremy Silman's How to Reassess Your Chess reminds us: you need to do one of three things when you have a Bad Bishop: exchange it off the board, move your Pawns so it's not a Bad Bishop, or move it outside your Pawn Chain.  The Slav does the last one and often later the first one - sometimes trading for White's Good Bishop.

Finally, the structure of the Slav Defense is much like the Caro-Kann and is sometimes a mirror image of the London System's basic setup, so you can understand all three better due to common themes.

All in all, it depends on your style and skill set.

Actually, it depends on terminology cuz Slav can be considered as a part of QGD. 

No it can't :/

That's an unhelpful description as the Slav and 2...e6 share little in common. Usually QGD means exclusively 2...e6.

Come back to the kindergarten in order to learn some basic things. 

https://www.365chess.com/eco/D10-D15_Queen's_Gambit_Declined_Slav_defence