For me, it's Chess Champions.
A variant I developed to entertain some friends and myself.
Which is the best chess variant ?
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my personal favorite will always be Fog of war because the strategy just feels more deep to me
Paradigm Chess 30 https://www.chess.com/variants/paradigm-chess30/leaderboards
.,,,,is hands down the best quality chess variant apart from the rest which is not really chess but only played on a checkered board some might be closely linked to chess and its laws but PC30 in fact improves upon the variant Chess 960. Check out the website www.paradigmchess30.com for comprehensive info on rules events and their streams on Twitch. Soon going to be big
PC30 is limited to only 30 random chess positions while adding Dragons that move like bishops and Xiangqi Horse (not jumping) in place of the original bishop which creates crazy dynamics that actually help with improving your standard chess calculation. For any chess lovers out there especially those fond of the romantic era of chess this variant is what you've been searching for all your life! 
Blindfolded chess.
I have tried different chess variants on several servers: chess.com, lichess, littlegolem, etc. I've tried some real serious ones, including correspondence tournaments, opening analysis, etc. If I don't consider Japanese chess, the most fun variant is CrazyHouse in my opinion. Basically every game ends with a fireworks display of hard to calculate combinations.
But the best variant is GrandChess. The clever initial position of the pieces, the conversion of the pawn already on the 7th rank, the cancellation of the castling make this variant probably the best version of chess. The larger chessboard allows two additional pieces to be added without the pieces feeling cramped as in Seirawan chess, for example. At the same time, there's nothing extra special that makes GrandChess essentially a completely different game than chess.
My personal favorite is Chess960.