I like Vileplume a lot, and while it's not the best ingame, this is one set I've always ran for it and it works pretty well :D
Vileplume @ Leftovers
EVs: 4 HP / 252 Special Attack / 252 Speed
Moonlight / Hidden Power Fire
A great set for in game, and probably its best shot at sweeping in gen III competitive play as well. With Sleep Powder, you can get a guaranteed chance to set up Sunny Day. Now, your Speed is doubled, and you get to abuse a 180 base power ( after STAB ) Solarbeam, as well as a Sun boosted HP Fire which covers Grass and Steel types. If this is ingame, you can ignore the EVs / Hidden Power if you wish. Moonlight is then the better option for lots of lovely healing in sun, allowing you to repeat the process.