Steelix has a crapload of Defense, but the problem is that it's got plenty of weaknesses as well, not to mention that it lacks the movepool to help that rating. I'd go with Weezing. It's only physical weakness, Ground, is nullified by Levitate, it has a ton of Defense as well, and it's got a nice movepool to help it be more than just a wall.
Weezing @ Leftovers
Trait: Levitate
Nature: Relaxed (+Def., -Spd.)
EVs: 240 HP, 200 Def, 68 Atk.
Sludge Bomb
Haze
Will-O-Wisp
Rest
You actually want the lack of Speed so that you can Haze to start.