Spartan Chess. Because end-games are the part of Chess that I like most, and there never are any symmetric dead draws in Spartan Chess. It will always be the battle between unequal material, each side having to exploits its particular strength and exploit the characteristic weaknesses of the opponent's material.
I am also intrigued by Chu Shogi, although I foresee that the length of an average game could be a deal breaker. It takes time to wear down your opponent when he starts with 46 pieces... But this Lion piece really offers an entirely new dimension to the game, introducing entirely new tactical concepts, such as 'hit-and-run' captures. Yet it feels quite familiar to Chess players in most other respects, featuring Rooks, Bishops, King and Queen.
I was wondering what your favourite chess variants are and why.