Two or more reasons.

Good question! Fischer springs to mind for obvious reasons, though I still admire his chess playing immensely. I also have a self-admittedly unfair dislike for Carlsen, simply because he's successful and I generally tend towards the underdog.

It's difficult for me to provide two or more reasons for disliking any chess player, though. I considered stating Botvinnik due to the politics surrounding him, but any criticism should probably be levelled at the Soviet Union and the Soviet chess federation. For one thing, Botvinnik managed to avoid signing the letter condemning Korchnoi after his defection.

I assume that Kirsan Ilyumzhinov plays chesss, and so I'll tentatively go with him for my least favourite chess player, for the following reasons:

- There is a strong possibility that his election as head of FIDE was due to corruption.

- I have questions about how he is running the organisation.

- There are far too many pictures of him associating with horrible people, eg. Muammar Gaddafi & Bashar al-Assad. This is alone should be reason enough to dislike the man.

Good answer.

I have an irrational hatred of Carlsen. I sleep better at night every time he loses a chess game. Topalov too.

I dislike Nakamura's grumpy, sullen aggressiveness enough always to root for the other guy. (He is a damn impressive player though!)

Wasn't it Matulovich who after killing a woman with his car said, "She was only a Bosnian"?

That's from http://www.amazon.com/Even-More-Complete-Chess-Addict/dp/0571170404

If you have it maybe you can add a detail or two. My copy is long gone.

Same guy dubbed JaDoube-ovich for his many illegal takebacks.

Even minus the vehicular homicide, the guy seems to have moral issues, no?

Hmm, does Deep Blue count? 

Oxford Companion says:: Although a non-smoker and teetotal, his anti-social behaviour leads to his being invited to few tournaments.

Anyone who beats me.  Because they suck.  And because they really suck.

same for me

Strange book.

wat does that mean?

Anyone who beats anand

There are some quirks I find irritating. Quick draws by Grischuk, lack of fighting spirit in general (Anand, Leko), rudeness (Nakamura, Bogolyubov), primadonnism (?) as exhibited by Fischer...

My least favorite chess player of all time is Tom M who I loaned

$200.00 to about 35 years ago and then disappeared from chess tournaments and never paid me back.

2nd worse Fischer [although he is also one of my favorites]

3rd Nakamura for playing a Ponziani line he did not understand [he is also a favorite]

My favorite is Kasparov.

Alekhine played in Nazi-run tnmts, along with Keres and Bogolybuv. He also wrote anti-Semitic tracts for the Nazis, or else let his name be put on them. As a chess-player he's still one of my faves, but as a human being he appears to have left something to be desired...like some integrity and humanity. Another world champ, Euwe, was also stuck in Nazi held territory during WW II but he refused to play in any Nazi tnmts.

But on the other hand, Fischer was also an anti-Semitic scumbag and sociopath who used people as long as they kissed his ass on a 24/7 basis and totally rejected them the instant they stopped doing so. He's also one of my fave GMs - but again,  only at the chessboard. Well, at least he didn't cooperate with Nazis, but he probably would have if he had had the opportunity to do so.

Tough choice here...could be a tie

I've never liked Botvinnik, he often strikes me as a pompous and humourless Stalinist. The composer Barulin was arrested for a politically incorrect joke, and wrote a defense of compositions as an art in themselves. Botvinnik had written that Barulin was "good for nothing", and he later died in the Gulag. In the 1980s Botvinnik collected texts for publication and chose to include and reprint the one where he declares Barulin to be "good for nothing". Not much sympathy or regrets there.

There's a lot of butts in chess, I guess.

kasparov cause hes a sore loser and was unfair to quite a few players. 

FIDE cause of how unfair they were to Susan Polgar so much so she quit competitive chess.