Pokemon I hate: bulky dark types or pokemon that always seem to win in stall battles. That list goes as following in terms of Most hated to Hated but not as much:
1. Mandibuzz
2. Sableye (both mega and rega)
3. Skarmory
4. Blissey (fuck you, 4th gen.)
5. Umbreon (no seriously, fuck you, 4th gen.)
so mostly bulky dark types.
in terms of just "worst" pokemon, I've got to go with noibat. It's so useless for a dragon/flying, and you're stuck dragging it around for 48 levels until it's useful. Of course, you could come back for it later in-game, but for a game that tantalizes you by dangling