Which chess variant is the best to improve your game in the traditional game?

The title and your post seem to be asking somewhat different questions. 1.) Title question: Whichever variant happens to highlight your weakest point in traditional chess, I'd imagine. I've heard games like bughouse can improve your attacking, 960 will improve your understanding of the openings if you can believe that, spartan chess is more similar to an endgame. 2.) Posted question: My personal favorite variant is Seirawan chess. Powerful pieces like Capablanca chess, but on a standard board size. Variable openings that thwart traditional lines, but with more inherent harmony than 960. It is immensely challenging (especially for a patzer like me) and just loads of fun to create your own strategy and openings and use new tactics that are impossible without the special pieces. Hope this helps!