Heh, I am definitely in the minority here, but for me it would be a tie between Dungeons of Dredmore and TOME. DoD I think has been unmasked over time as a basically stupid and boring game with a dumbass Monty Python motif, but TOME is very well-loved by a lot of people.
The numbers would indicate that TOME is just great and I'm an idiot. The game has an active and involved community, and they love the dev (and they should! he's a cool dude who will turn your character into a pirate sometimes.)
The reason I don't like TOME is because it seems like it lacks heart or purpose. It started as a *band variant and mutated into something that resembles... I dunno... The League of Legends extended Multiverse? "In the time before the Spellblaze, blah blah blah, now mages are hunted except they actually aren't..." The plot is so meandering and irrelevant, and the game is so repetitive; you perform the same sets of missions again and again and again. It's a grindfest that lacks much in the way of discretion. And the nature of combat, wherein you lose 400 health, regenerate 300 health, get five status debuffs, and seven buffs over the course of a single round... it's just not particularly exciting to me. It seems excessive, yet meaningless.
I guess I'd call these the worst (for me) because they actually managed to make a negative emotional impression on me, and as they say, it's a thin line between love and hate. I've played plenty of broken or generic roguelikes that made no impression on me whatsoever. I guess empirically those would be the worst (as Ancient mentioned)