A poison type's worst nightmare is a Steel type.
Due to the fact that poison attacks do no damage - literally no damage to Steel types who are invulnerable, Posion types will be able to do no damage with their main moves, and will be forced to rely on other coverage moves to win
Alternatively a ground type would work well. Ground type attacks are 2x effective against Poison types, so can often KO them and make life really hard =)
Psychic types are in the same boat as Ground types overall
So some excellend examples include;
Graveller, Nidoqueen, Nidoking, Rhydon, Onix, Steelix, Rhyhorn, Golem, Kadabra, Alakazam, Starmie, Mew and Mewtwo.