Hmm, this is hard to say. Back in the first games I really hated Poison type moves and I never had any Poison type Pokémon, so I never really got used to them. I got some Poison type Pokémon in later games though and if I'd have to choose between their moves, it'd probably be either Sludge or Toxic. I know that Sludge in itself is a really strong move and can be a real pain in the butt with its possible effect of poisoning ones Pokémon, so naturally it's a great move to have yourself. Toxic instantly poisons the foe and its damage keeps increasing during every turn it's in effect, so it's great to use against stronger opponents that could take some time to beat, though it's really annoying to be fighting against, naturally. A last move would probably be Poison Jab, though. I don't know very much about the move itself, since I first learned about it while playing PO. It was perfect with my Ursaring, and often damaged my opponents a lot, but I don't really know anything about the move itself. >_<