There are only a handful of good Poison-type Pokémon, and even then they're gimmicky. All but Drapion have this annoying weakness to the very common attack, Psyshock – and with the overabundance of Psychics like Gardevoir, Latios and Mega Alakazam, your options are limited. Here are a few Poison Pokémon that rise above their toxic brethren to become the superiors of the inferior.
Venusaur
Easily one of the best Poison-types there is, Venusaur is arguably the best Sun sweeper and also has a Mega Evolution.
Venusaur@ Black Sludge
Tier: OU
Ability: Chlorophyll
Nature: Modest
EVs: 232 HP / 252 SpA / 24 Spe
- Giga Drain
- Synthesis
- Sludge Bomb
- Hidden Power [Fire]
Get the sun up, then sweep to your heart's content. Giga Drain for recovery and STAB, Sludge Bomb is good Poison STAB, and Hidden Power Fire is boosted by the Sun and batters Steel-types. Synthesis is a lifesaver, recovering 50% of you max HP when used in the Sun. Chlorophyll Venusaur is tiered in OU, it has been used successfully on many Ubers teams.
Venusaur@ Venusaurite
Tier: OU
Ability: Thick Fat
Nature: Bold
EVs: 252 HP / 252 Def / 4 SDef
- Giga Drain
- Leech Seed
- Sleep Powder
- Sludge Bomb/Hidden Power [Fire]
Mega Venusaur is much more defensively inclined than regular Venusaur. It's Ability negates two of its weaknesses, leaving only Flying and Psychic. Giga Drain is for recovery. Sleep Powder is to put away threats. Leech Seed deals passive damage and also heals Mega Venusaur. The final moveslot is either STAB or coverage – without HP Fire, Mega Venusaur is completely walled by Steel-types, particularly Ferrothorn.
Gengar
Gengar has to be played carefully, removing any threats before the sweep, but once let loose, Gengar can win a match almost single-handedly. Everyone's favorite pudgy ghost came back with a vengeance in XY, with the release of the OP as heck Mega Gengar.
Gengar@ Life Orb
Tier: OU
Ability: Levitate
Nature: Timid
EVs: 252 Spe / 248 SpA / 8 HP
- Shadow Ball
- Sludge Bomb
- Focus Blast
- Taunt/Thunderbolt/Energy Ball
Gengar's sole purpose in life is speedy Special sweeper: once unleashed, he is nigh unstoppable… unless the opponent is faster. That aside, Gengar gets fantastic coverage moves. Shadow Ball is powerful STAB, as is Sludge Bomb. Focus Miss hits like a truck, when it hits. Taunt can be used to shut down walls and slower leads, while Thunderbolt provides extra coverage. Energy Ball surprises Rotom-Wash.
Gengar@ Gengarite
Tier: Uber
Ability: Shadow Tag
Nature: Modest
EVs: 252 HP / 252 Spe / 4 SDef
- Perish Song
- Disable
- Protect
- Shadow Ball/Sludge Bomb
Mega Gengar, the ultimate Perish Trapper. The strategy is simple, yet deadly – trap the enemy with Shadow Tag, Protect every other turn, Disable their attacks, and switch at the last second and watch them suffer. Shadow Ball and Sludge Bomb are STAB so that Mega Gengar isn't complete Taunt bait, but only one can be chosen.
Tentacruel
Tentacruel isn't so much about power as it is about usefulness. It also follows the trend of the best Poison-types coming out of Gen I.
Tentacruel@ Black Sludge
Tier: UU
Ability: Liquid Ooze
Nature: Bold
EVs: 252 HP / 252 Def / 4 Spe
- Sludge Wave
- Scald
- Rapid Spin
- Knock Off
Tentacruel is probably the best offensive Spinner, bar Excadrill. It can switch in on Stealth Rocks with little damage and spin them away. It also automatically removes Toxic Spikes. Sludge Wave and Scald are nice STAB attacks, offering a chance to poison or burn, respectively. Rapid Spin is to, well, spin, and Knock Off is a great utility move. Event though Tentacruel is tiered in UU, it is a common sight in OU.
And that's it for the most useful Poison-type Pokémon. Funny, they all seem to come out of Generation I. Honourable mentions are Crobat, Toxicroak, Nidoqueen, Scolipede and Mega Beedrill.