IloveScotch is really good! I love playing him/her!
Toughest player to play against?

I came across some good players (2000+) recently
Cyanriddle, ConcorMaclaud, coronalipsis and Hydrazine

I am in te low 1800 myself , so you just have to be lucky to be paired against strong opponents I guess

In rated about 1910 in Rapid, and the highest rated player I have beaten is Thats_Gonna_Hurt, 3-1 head to head.

I played Luizzy, the current leaderboard #1 (as of June 1, 2021) several times.
What fuels his insanely high 2450+ rating is excellent reasoning of the enemy position, mastery of intelligence, accurate guesses even when he lacks information, sharp tactical play exerting tremendous pressure, solid master-level knowledge of regular chess as a basis, and perhaps the most annoying of all, bluffing.
He knows when your king / queen is the only defender of a piece, and gets away with stealing your pieces nearly every time by playing with your psychology. He uses the common assumption of Fog of War players that an enemy piece is well defended unless proven otherwise against them.
This culminated in his match series against vsaxena (the old leaderboard #1, now over 200 points behind Luizzy) where he absolutely crushed him. Vsaxena has a very sane, solid style and is ruthless when he spots any enemy weaknesses. Try play vsaxena yourself, you won't survive very long. I did commentary on their match.
The scary thing about kibitzing Luizzy games is his accurate markings and arrows, where he nearly always get a very accurate picture of the enemy position very early in the game, and never let go of that information superiority. Even vsaxena feels naked when playing him. The only way Luizzy practically loses is when he gets outplayed by the chess or makes a false reasoning, which happens occasionally. He is the kind of insane guy who nonchalantly plays queen takes pawn, banking on his assumption that your pawn is completely undefended. It's very hard to prove him wrong.
I did beat him twice, in https://www.chess.com/variants/fog-of-war/game/18187173 and https://www.chess.com/variants/fog-of-war/game/18186806 respectively. The first one he blundered a queen on move two.
I'd say maybe cooldude, theres another 2000 rated player i remember losing to a few times but I forget the name.