Venusaur is the first fully evolved pokemon in that National Pokedex and intrigues me very well. Its ranking is BL. Its type is Grass/Poison, and stats indicate for a special wall. Here are some good sets:

Special Wall
This is probably the standard set for a Venusaur.

Nature:Calm
Item: Leftovers/Black Sludge Evs: 252 Spec.Def 252 HitPoints 6 Spec.Atk
Moves:
LeechSeed
SleepPowder/Toxic
EnergyBall/SludgeBomb
Protect/Synthesis

This is one of my favorite venusaur sets. Leech Seed is the only must on this set. Other then that, choose your status move, Toxic or Sleep. Most prefer sleep, but if ur running Protect I reccamend toxic. Energyball or SludgeBomb is based on your teams needs.

Counters: Any good Phyiscal Sweeper with a fire/ice/flying move will easily KO this thing. Its rather slow, so anything faster then it with a good Physical Attack will probably be a good counter.

Choice Specs
This set puts you on the offense on your opponent on the defense.

Nature:Modest/Timid
Item:ChoiceSpec EV's: 252 Spec.Atk/252 Speed/6 HitPoints
Moves:
EnergyBall
LeafStorm
SludgeBomb
HP Water

Obviously, a powerhouse set. HP Water eliminates fire weaknsesses, Sludgebomb for other grass types, and E-Ball/Leaf Storm for STAB Power. Timid or Modest is simply opinion, although Venusaur is rather slow so Modest will probably be your best choice.

Counters: Again, anything faster will work. But since this set does have the defenses of the previous set, even a good special attack will KO. This sets particular weakness comes from Azelf, which has 2 moves that knock it out completely.

Special Tank

Nature:Modest/Calm
Item:WiseGlasses/Leftovers/BlackSludge Ev's:252 HitPoints/178 Spec.Atk/80 Spec.Def
Moves:
EnergyBall
LeechSeed
SludgeBomb
Protect/HP Water

A mixture of the above to set. Protect or HP Water is simply a matter of preference, a lil extra offense of a little extra defense. The EV's ensure tankability and at the same time some damaging power, and Leech Seed ups the anti. In terms of items, again, preference over a lil extra offense of defense, although I reccamend Wise Glasses for a lil extra power as leech seed pretty much covers all your healing:

Counters: This set is arguabley the best grass pokemon in the game. It takes alot of hits, disshes out alot of damage, but because of its part poison status its massive library of weaknesses send this to a halt. The HP and SPec.Def ev's only hold out for so long, so any Physical Fire/Flying/Psychic/Ice move will KO.