Battle Moveset

A Good Moveset for Venusaur would have to be this:

Attacks:

Leech Seed
Attract
Toxic
Magical Leaf

Items Attached:

Salac Berry

Preferred Nature:

Calm

Strategy Using Venusaur

Venusaur has a few possible movesets it can use and learns a wide variety of moves that can provide annoyance.
This moveset however, is designed to be annoying (I would've given you the Sunnybeamer set, but since Tangela has just been done, I don't want to repeat that moveset) and will also help replenish some health if you do get hit by an attack.
Since Venusaur has his stats dispersed somewhat evenly, it can be of use as a tank too. However, Venusaur's best stats are its specials, which allows for it to be the aforementioned Sunnybeamer/Annoyer.
You may require some precognitive thinking if you wish to use this moveset, try to avoid a majority of steel types that may be thrown into the battle.
To avoid further bafflement and confusion, I shall explain the workings of this set, if you don't know it already. Toxic and Leech seed, the coalescence of two perfectly helpful moves. Toxic, the Health pointer drainer which augments with each turn, dealing a greater amount of damage.
Leech seed, the move that sponges of your opponent's HP, confiscating that much needed HP if it wishes to survive any longer. As for the other moves, of course Attract makes perfect sense, it will beleaguer your opposition with the ability of preventing from his Pokemon from attacking.
Magical leaf, the 100% accurate move, no matter what kind of accuracy lowering effects are used. The exactitude of this move should provide you with quite a bit of help if Venusaur ends up becoming your last Pokemon.
This is such a Puissant Pokemon, with a various potential movesets that each, in there own right, will do Venusaur proud.

EV Corner:

Max out the EVs in Special Defense for Venusaur with any remaining in HP & Defense

Strategy Against Venusaur

Venusaur' Defenses are both above average. However it's Physical is lower, so a Physical volley should work. Else a Fire, Ice, Psychic or Flying type volley would take it down in a few turns.

Contest Moveset

A good Contest Moveset for Venusaur would have to be this for the Cool Contest best with Lonely, Adament, Naughty or Brave Nature:

Attacks:

Leech Seed
Sunny Day
Solar Beam
Hyper Beam

Items Attached:

Red Scarf

Strategy Using Venusaur

If you do the attacks in this following order you should have very few problems; 1st - Leech seed, 2nd - Sunny day, 3rd - Solarbeam, 4th - Leech seed, 5th - Hyper beam

Locations in Games

Ruby/Sapphire Trade from FR/LG Colosseum Trade from FR/LG Fire Red/Leaf Green Evolve Ivysaur

Animé Appearences

Venusaur has had a few Animé Appearence. First it was a Robot in the Giant Pokémon Island Amusement Park. After that, it was leading the Bulbasaur evolution Ceremony. A Trainer, who went to New Island had one called Brute Root which ended up getting Cloned by Mewtwo and lived with all the Pokémon Clones later in Johto. The Orange Champion, Drake, also had a Venusaur in his team that he used to battle Ash's Tauros. An Artist, Gan Gogh, who was searching for Remoraid for pictures had one too and used it against Team Rocket. Team Rocket used one in the Battle Park in a battle against Ash's Cyndaquil in an elaborate plan to get Pikachu. One also lived in a forest ruling over many Grass Type Pokémon. This forest was classed as forbidden by the village folk, however May got kidnapped by a Skarmory and landed in it by a Bulbasaur. The Venusaur had a hand in defeating Team Rocket and protecting the forest Episode 017: Island Of The Giant Pokémon! Episode 051: Bulbasaur's Mysterious Garden! Movie 1: Mewtwo Strikes Back! Episode 114: Enter The Dragonite! Special 2: Mewtwo Returns! Episode 182: Houndoom's Special Delivery! Episode 187: The Light Fantastic! Episode 250: One Trick Phoney! Episode 349: King of the Forbidden Forest! Venusaur!