What is the hardest opening for average players?

Very theoretical openings, as the rate of flow of information will be too much, can't have a repertoire that needs adjustment all the time.
Very complex openings with more pawn structures that one can hope to learn and understand.
Openings where lots of lines end in positions that cannot be evaluated classically.

The Ruy Lopez as well as the Najdorf are without a doubt the most insanely difficult systems to master.

The mainlines of the most popular openings, those being: English, QGD, Slav, NID, QID, Gruenfeld, KID, Morphy Ruy Lopez, Berlin Ruy Lopez, Italian, French, Caro-Kann, e6 Sicilian, and d6 Sicilian.
I have a few honorary mentions of openings that are also incredibly difficult to play for average players: Pirc (the mainlines are essentially a series of only-moves for Black, and the sidelines are nigh unviable), Advance Caro-Kann (slowly becoming the new mainline, with the theory to back it up), Three Knights QGD (leading to incredibly critical defenses such as the Ragozin, Semi-Slav, Semi-Tarrasch, and Neo-Orthodox), and any gambit/trappy defense with either enough theory to be irrefutable by non-titled players (such as the King's Gambit, the Scotch Gambit, or the Goring Gambit) or one which that sacrifices multiple pawns/pieces for insane compensation if accepted (such as the Pin Sicilian, Delayed Alapin Sicilian, Canal Scotch Gambit, Double Muzio King's Gambit, Double Pawn Sacrifice Goring Gambit Scotch, et cetera).

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.

No the Najdorf is worse I still wouldn't play it because White can play almost anything against it, and the plans for Black are complex, and even as White you have to be good at attacking.

The Ruy Lopez as well as the Najdorf are without a doubt the most insanely difficult systems to master.
And the Sicilian is a picnic!
It does have ants.

knowing everything black can do in the catalan is an absolute pain
sicilian is not that bad as in the closed sicilians you adopt the same setup each time, and in the open sicilians you dont need to know 10+ variations of theory
playing the benoni makes you need to know the many setups white has and how to play in each of them
the grunfeld is an absolute migraine to remember if you don't have natural remembering ability...
these are the reasons the people you play against only play the london/kid setups in their games, but instead they are told that this applies to every opening that exists (i.e gotham chess) and are recommended to play system openings/much less theoretical openings