Last generation, Venusaur was outclassed by Roserade, who has the same typing, but more Speed and Special Attack. Venusaur has risen above it now though, due to what it got in the fifth generation. First of all, it got Chlorophyll, which brings its mediocre speed stats to high levels. Next, it got an improved Growth, which gives it a +2 boost to both its Attack and Special Attack when in the sun. Lastly, the advent of Drought Ninetales gives Venusaur endless sunlight to use the aforementioned traits. Although it sounds crazy, it can even function in Ubers, as Groudon is a very prominent force in the tier. The changes in BW2 also went in its favor: It got Giga Drain and Seed Bomb from the move tutors, meaning they can be used with Chlorophyll. The only real flaw with Venusaur is its movepool, which mostly consists of Grass- and Poison-type moves. It also has a somewhat lackluster Defense stat, so it struggles against 2 of the most common priority moves: Bullet Punch and ExtremeSpeed. Overall Venusaur is a great Pokemon that should always be considered when making a sun team.
Abilities
Overgrow:The power of Grass-type moves increase by 1.5× in a pinch. Not too useful, as Venusaur's main selling point is Chlorophyll. The only reason you would want to use this is if you really want Power Whip and Leaf Storm, but then you would be better off using Tangrowth or Roserade.
Chlorophyll: As mentioned before, Venusaur's main reason it improved so much. Allows Venusaur to be the fastest unboosted Pokemon in OU, making it an excellent sweeper.
Special Growth
Item: Life Orb
Ability: Chlorophyll
Nature: Modest / Timid
EVs: 4 HP / 252 SpA / 252 Spe
- Giga Drain / Grass Knot / SolarBeam
- Hidden Power Fire
- Sleep Powder / Sludge Bomb
- Growth
This is Venusaur's most effective set. The First moveslot is completely up to the player. Giga Drain is generally the recommended option, as it recovers Venusaur's health while dealing respectable damage. Grass Knot can be used to hit Tyranitar, Suicune, and Groudon harder. SolarBeam is viable as Venusaur is almost always used in the sun. Just make sure that the opponent's weather inducers are eliminated before you use it, since the opponent can trap Venusaur for a turn if they switch in when SolarBeam is selected. Hidden Power Fire is usually a necessity so Venusaur can take down Steel-type without leaving it to its teammates. Sleep Powder can eliminate an opponent's Pokemon for the rest of the match, while Sludge Bomb provides secondary STAB and more coverage. Growth is the staple of the set, boosting Venusaur's Special Attack to 500+.
Physical Growth
Item: Life Orb
Ability: Chlorophyll
Nature: Lonely / Adamant
EVs: 252 Atk / 4 SpA / 252 Spe
- Seed Bomb
- Return / Earthquake
- Hidden Power Fire / Sleep Powder
- Growth
This is another approach at a sweeper, except with more coverage. Seed Bomb hits reasonably hard after a boost. Return gives good coverage with Seed Bomb. Hidden Power Fire is recommended in the last slot, as Seed Bomb and Return are walled by the ever-present Steel-types. Sleep Powder could still be used if you have a teammate that can handle these threats along with the handful of other Pokemon that wall this set. Earthquake could also be used over Return to hit Chandelure. In this case, Sleep Powder is more viable over Hidden Power Fire, as Earthquake hit Steel-types super-effectively.
Swords Dance
Item: Life Orb
Ability: Chlorophyll
Nature: Jolly
EVs: 92 HP / 252 Atk / 164 Spe
- Seed Bomb
- Sleep Powder
- Earthquake
- Swords Dance
This set is designed to catch it's normal switch-ins off-guard, particularly Dialga and Steel Arceus. Seed Bomb is the main STAB of choice, OHKOing Jellicent, Kyogre, and all but the most defensive of Groudon after a boost. Earthquake can surprise Poison-types, particularly Toxicroak and Tentacruel.
Mixed Growth
Item: Life Orb
Ability: Chlorophyll
Nature: Jolly
EVs: 92 Atk / 252 SpA / 164 Spe
- Grass Knot / Giga Drain
- Sludge Bomb / Sleep Powder
- Earthquake
- Growth
Rather than being Physically-based, Venusaur is also capable of running a set that doesn't focus on either Special or Physical attacks. Grass Knot OHKOs both Kyogre and Groudon after a boost. Giga Drain is still an option though, to give Venusaur more longevity. Sludge Bomb can hit more targets for neutral damage, but is notable as it stops Venusaur from being setup bait for Shaymin-S. Sleep Powder immobilizes Pokemon for possibly the rest of the match, so it could be used over Sludge Bomb. Earthquake is one of this set's selling points, surprising Dialga, Heatran, and Jirachi.
Other Options
SubSeed is viable, and it stands out as Venusaur is probably the only common Grass/Poison-type to use it. Outrage can catch Latios by surprise, but isn't too good as it lacks STAB. Venusaur also has other moves including Power Whip and Leaf Storm, but it must forgo Chlorophyll for them. Like most Pokemon, it got Sleep Talk in BW2, making a mono-attacking set with Rest, Sleep Talk, Growth, and Giga Drain or Grass Knot viable, to heal paralysis, which really hinders Venusaur. Double-Edge could be used over Return, but the recoil discourages this as Venusaur generally wants to stay in as long as it can. Petal Dance is Venusaur's most powerful move, but due it's side effect of being locked into it combined with the fact that Grass-type moves are resisted by six types, it's discouraged. Block, Knock Off. and Toxic round out Venusaur's support movepool, but don't see much use.
Counters
Venusaur is very difficult to counter due to Sleep Powder, but once Sleep Clause is activated, it's more manageable. Heracross comes to mind, as it can absorb Sleep Powder, but must get lucky, as only Megahorn will OHKO. Another way would be to get rid of Venusaur's sunlight. Choice Scarf Abomasnow does this best, as it can come in on anything but Sludge Bomb and Hidden Power Fire, and hit Venusaur with Blizzard. Tyranitar can also do this, but will lose to Venusaur who carry Seed Bomb, Grass Knot, or Earthquake. Heatran works well as it resists and is immune to Venusaur's STAB moves, and it benefits from the sun, and works even better if it's carrying an Air Balloon to avoid Earthquake. Paralyzing Venusaur severely hinders its sweep. Whimsicott and Thundurus work the best at this, as they have Prankster to give it a priority Stun Spore and Thunder Wave, respectively.
