I have to agree with the above in regards to The Poopsmith. There's really nothing all that likable about him. Granted there's nothing all that unlikable about him, either, but nothing to make me really adore him as a character.
I'm also not a huge fan of Pom Pom, partially because he's so
cool that he can be kind of snobbish (for example toward Homestar or Strong Bad). Of course, I'm not saying that those two don't
deserve it sometimes, I'm just saying that it makes me like Pom Pom less. I don't really
dislike him, per se, but he's just not one of my favorites.
Now, getting out of the main cast, I have to say that I truly dislike The Homestar Runner (the old-timey one). Something's just genuinely unsettling about him that I can't quite express. He kind of creeps me out. He'd probably be my least favorite character in all of the Homestar Runnerverse.
(Oh, and, for the record, Strong Sad is probably my favorite character.

)