Crazyhouse, for sure. Although I'm not very good at it, this variant sure is a nice break from regular chess on those "off days". Bughouse is a wonderful variant OTB as well, but I cannot stand to play it online.
Here is a write up on Crazyhouse, or ZH chess.
http://www.associatedcontent.com/article/1331874/crazyhouse_chess_overview_of_the_chess.html?cat=19
Crazyhouse sounds very interesting, I'd like to try that. Grasshopper chess and gothic chess could also be fun to play but I would have to say that crazyhouse gets my vote for now.
Probably Bughouse, but I play a wide variety of variants. Just started some Stanley Random games, which is pretty odd.
I love bughouse!!!!!!!!!!!!!!!!!!
W00T!!! 3 bughouse fans in a row now!
Seirawan (or 'Sharper') is great fun - esp. if you enjoy playing your Knights. The Elephant and the Hawk also look very cool. If you play in a coffee-house, plonking one of those down on the board usually raises some eye-brows at nearby tables. :)
Boom...Atomic
Exchange, Bughouse
Actually, Progressive Chess is probably my absolute favorite, but very few folks know how to play it. I'm going to write a Web article on it and when it publishes I'll link all you good people to it!
Any other progressive fans? :)
Ghost chess, it required three people to play live.
You two sit next to each other, with a cardboard blocking the other person's board so you can't see what your opponet it requires a lot of mental thought to figure out what your opponnent is doing, the third person has a full board and tells you whether the move is leagal or not.
Any purists here? Forget about variants. Chess is great as is.
The point of chess variants is not for them to be better than standard chess, but to be another way of playing. Chess is great as it is, but chess variants are simply different ways of playing one great game.
I've only played various forms of Shuffle Chess & Suicide Chess. I like shuffle chess the best. I didn't care much for Chess960 at first on account of it's funky castling rules but I'm warming up to it somewhat.
Basically I like chess & I can't be arsed to learn a completely new game (like Seiriwan chess) at this time. Shuffle chess is an exception because it's the same game with dynamic potential right out of the box. 
If you know how to play chess, it takes about 30 seconds to learn how to play Seirawan chess. The bigger problem is the requirement for the two new pieces.
The Hawk can move either like a Knight or a Bishop.
The Elephant can move either like a Knight or a Rook.
You play on a standard board, by the standard rules, except whenever one of the normal pieces (not pawns) is first moved, you have the option to put one of these two bruisers in the vacated square. (When castling you can put ONE of these two pieces on either the King's old square or the Rook's old square.)
If you move all of your regular pieces without ever replacing any of them with the special pieces, you miss your chance to use them at all.
You cannot block a check by dropping one of these pieces in the place of a piece that moved out of a pin (illegal).
That's pretty much it.
Although classic chess will always be #1 for me, these games can be a fun change of pace. Sometimes all the extra power on the board in the early part of the game makes for a lot of fireworks, the Seirawan pieces get traded off, and your left in a totally normal looking middle game.
There's just something about a piece that can shoot all the way across a board and, once there, attack like a Knight. :) Good times.
Since Seirawan is being talked about, he was at the 1988World Chess Festival in St. John N.B. promoting a game called choiss. All the pieces are the same, but the board is configured differently. Each of the 64 squares are individual pieces. You start off with 4 squares on the table (2 white and 2 black). Each player has the remaining squares, one has the white squares and one has the black ones. Each player then adds a square to the board (you must adhere to a checker board pattern so that white squares can only be attached to black squares and vice versa). It is very interesting as the board is always different. I don't recall how pawn promotion works, or castling. The pieces would be set up by each player on his side of the board, however he would like to. A screen would be set up in between so that you couldn't tell how your opponent was setting up his pieces.
transfer and atomic
Ralph Betza's Chess with Different Armies.
I dislike Seirawan Chess. They put too much power at 8x8 board.
So which is your favourite chess variant or do you only prefer standard chess?
I am quite new to chess variants so I don't have a favourite yet but I guess marseillais chess would be the one that I like the most because I actually used to play it as a kid when I didn't know the rules :P The idea of the variant is that you get two moves every turn.