Grunfeld and Modern Benoni are both hard to play if you don't know the lines.
What is the hardest opening for average players?

Either the Sicilian Defense or the Grunfeld. The Sicilian, in context, is probably either the Najdorf or the Sveshnikov. I’ve heard from some that the Sveshnikov is somehow harder, so I’ll take it that the Sveshnikov and the Grunfeld are the hardest openings to learn, especially because the Sveshnikov has somewhat unintuitive moves for both sides. The Ruy Lopez is a bit hard, but it’s really not that bad. The sidelines are usually simple and you don’t have to learn hoards of theory like people say you do. If you play the Exchange Variation, it’s even better!
I’ve heard that the Grunfeld is an absolute pain to learn, but I don’t actually play it so I wouldn’t know. Nimzo-Indian gang fr fr
The Sveshnikov is relatively simple compared to most openings, but compared to the Najdorf? My Sicilian recommendation used to be the Sveshnikov because it was so simple, and even though the theory required to play the Najdorf isn’t as dense as people say it is, it is still one of the most theory dense openings out there.

The Sveshnikov goes against some beginner principles and requires attacking skill so I would NOT play it at begniner level

The Sveshnikov goes against some beginner principles and requires attacking skill so I would NOT play it at begniner level
Exactly my point. It has counterintuitive moves an average player would struggle to pick up on.

Sveshnikov is somewhat bizzare.... U need superb attacking skills and need to understand initiative deeply in other to win because u are playing with a lot of weaknesses
Yeah. It’s crazy how much needs to be sacrificed in order to gain the initiative from the opening for black. Black gets weaknesses on a6, d6, d5, and backward pawns on a6 and d6 just for an attacking game.

That’s exactly why the Sveshnikov is awesome! And that’s why it works out so well. The hardest opening to pick up the nuances on and stuff might be the Ruy Lopez. But the grunfeld and Najdorf are hard as well. Maybe kings Indian because of mountains of theory but meh not really

Yeah, the Sicilian is what first comes to mind for me, since it's very theoretical.
If I were to pick a specific variation of the Sicilian, maybe it would be the Dragon. You have to play very precisely against the Yugoslav Attack.
Good shout. I'd go along with this as a ex-long time Dragon player. Loved playing it but it is a real minefield. I now go 1. ... e5 and aim for a Closed Ruy Lopez, while hoping I don't get bored into submission by a Giuoco Piano or anything other than 2. f3. Tried the Pirc for a while but it's hard work getting counter play in many variations.

As natural as it looks, Pianissimo is pretty damn hard to play as, and against.
Really? I never found I had all too much issue on either side of it. I used to use it to cool off as White, and I found I actually enjoyed it a decent amount, but I didn’t like the absolute lack of tactical possibilities for so long. I like having a mix of positional and tactical play but there is NOTHING in the Pianissimo until pretty much every single piece is developed. From there though it’s perfectly fine and I scored quite well in my time with it.

Sveshnikov sicilian
white has tons of options and can avoid this easily, or he could deviate from the main line quite easily and you may be on your own, you also have to really know what youre doing in order to survive + you must be a tactical god

As natural as it looks, Pianissimo is pretty damn hard to play as, and against.
Really? I never found I had all too much issue on either side of it. I used to use it to cool off as White, and I found I actually enjoyed it a decent amount, but I didn’t like the absolute lack of tactical possibilities for so long. I like having a mix of positional and tactical play but there is NOTHING in the Pianissimo until pretty much every single piece is developed. From there though it’s perfectly fine and I scored quite well in my time with it.
The reason you gave is why most of my opponents are handling it wrong most of the time I think, trying to go for attacking when the position isn't ripe yet. My experience so far is that people mishandle this opening a lot though. I rarely ever play it as white so I can't really speak on it.

As natural as it looks, Pianissimo is pretty damn hard to play as, and against.
Really? I never found I had all too much issue on either side of it. I used to use it to cool off as White, and I found I actually enjoyed it a decent amount, but I didn’t like the absolute lack of tactical possibilities for so long. I like having a mix of positional and tactical play but there is NOTHING in the Pianissimo until pretty much every single piece is developed. From there though it’s perfectly fine and I scored quite well in my time with it.
The reason you gave is why most of my opponents are handling it wrong most of the time I think, trying to go for attacking when the position isn't ripe yet. My experience so far is that people mishandle this opening a lot though. I rarely ever play it as white so I can't really speak on it.
Ahhh that makes sense. I’ve seen many beginners make mistakes in it, so you’re most likely correct in not recommending it to beginners because beginners don’t do well if they don’t have a clear plan or a clear target, but the Pianissimo is a good one if they learn it well

The most complex opening is the Silican, but the average player is 600-700. So there are a lot of openings that are complex at that level.

The siccilians isn't hard due to the fact that scheveningen, taimanov, and kan exist. Those siccilians are eazy to understand, "Don't create weaknesses, play d6, and then d5." Is the best way to describe them. And for the Indian ones and the grunfeld, Eazy, Just 2 or 3 concepts and you're good to go. Ruy lopez is maybe hard, But after studying it for only 1 day, They can already play like a GM!
Either the Sicilian Defense or the Grunfeld. The Sicilian, in context, is probably either the Najdorf or the Sveshnikov. I’ve heard from some that the Sveshnikov is somehow harder, so I’ll take it that the Sveshnikov and the Grunfeld are the hardest openings to learn, especially because the Sveshnikov has somewhat unintuitive moves for both sides. The Ruy Lopez is a bit hard, but it’s really not that bad. The sidelines are usually simple and you don’t have to learn hoards of theory like people say you do. If you play the Exchange Variation, it’s even better!
I’ve heard that the Grunfeld is an absolute pain to learn, but I don’t actually play it so I wouldn’t know. Nimzo-Indian gang fr fr