I've thought of running this on a Sun team, along with Heatran, to set up the sun at +1 priority. I'm used to doing offensive 'Mons, so I've wondered how this set would work:
Stay Puft
Bold - Prankster
252 HP/100 Def/156 SpDef
@Heat Rock
Sunny Day
Taunt
Substitute
Leech Seed
Basically, I plan to switch in on a wall, Taunt it and try to set up a Sub, sun and then Leech Seed the switch-in before letting Heartran take the move meant for this lil' guy, since it resists everything that's super-effective on Grass, and is even immune to Fire (And Poison, not that it matters...). And if you're wondering about the lack of Encore, well, I'm doing this in the game and I bred my Cottonee before checking to see whether Encore was an Egg move. Good thing I was a slot short anyway...
And as for the weird EV spred, I have no idea, honestly. I shoved 100 vitamins down its throat in each of the two Defense stats, then hoped for the best. So currently, it has 100 in Def and SpecDef, while I'm going for the max HP. Yeah...
EDIT: Also considered Tailwind for support, but... There's no room for it...