hardest opening

According to me, in my perspective, it’s non-other than Najdrof[sicilian]…If you ask why? I guess that you know the answer too…It’s so vast and the theory is a lot, A-LOT means A-LOT…To play it from black, white has almost 20 different ways to play against it…and preparing it is not so easy…Also The King’s Indian Defense [which is 1.d4 Nf6,2.c4 g6] it’s way more aggressive,and the one who does a mistake has his consequences. And if You ask from white,I am not sure of it,But maybe the English,It’s positional but yeah according to me it is difficult to learn [^_^] ummm…well that was a different part….And I hope You found my answer a Helpful….

Hi

Probably a hypermodern opening like the Pirc

Seems neat, I’ll check it out

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Some lines in every openings are very theorical and hard but some openings like najdorf results black in lack of space and very difficult play to hold for black. In the other side white with just little theory knowledge could beat black.

And some others are ruy lopez because it will lead to very complex positions with SO MUCH theory and whoever does one mistake will be suffering in a losing position.

And so is the grunfeld because white gets a mobile center and have easier play but it also has so much theory

I think that the Italian is the hardest to play. 
why? White can play on the queenside, in the center, or on the kingside. Moreover, black can also play on all three fronts. 

the first 8 or 10 developing moves for white are about the same regardless of where white intends to play. But the variants, subtle nuances, move orders, etc. make it one of the hardest to play well. If you watch grandmaster commentary during tournament games, they say the same. 

actually, I don’t think that the Najdorf Sicilian is that hard to play. There are lots of move orders, attacks, sacrifices, etc. that white can introduce. But black’s set-up and avenues of counterplay are generally the same whatever white tries.

i played the Najdorf as a kid when I was learning chess and I am glad that I did. Sure, I got blown out of the water a few times, but that is how you learn.

For me, personally, it's probably 1.nf3.

Against 1.d4 I play 1...e5? (The Englund) because I like attacking chess, and want an open board very quickly. It's playable in blitz (at least at the 2000 level - and in fairness, even Aman Hambleton plays it from time to time at the 2800 blitz level). But nf3 prevents 1.e5 technically, so getting into a more traditional closed 1.d4 position is more likely in that move order.

If you're gonna play an early attacking opening against 1. d4 you really need to play the sicilian against 1. e4, so that you can play the sicilian invitation vs. the reti and the reversed sicilian vs. the english. It doesn't really work to combine e4/e5 and something like the englund gambit in a repertoire. Unless you're just content with surrendering much hope of dealing with the Reti. I suppose something you could try is to play a symmetrical reti but this still seems dubious.

Of openings I've tried, the Vienna Gambit, because exposing the f-file puts a bullseye on your king while you try to hold the initiative and push offense.

Hardest to play against, the slow positional-type people who play the Hippo and are good at tactical defense. I'm getting better at spotting key pawn breaks, but if it's closed and they are just shuffling behind the line without exposing a weakness, that's tough to break.