Hard to decide. WishBliss vs Miltank would end up in a stall fest. Miltank with Thick Fat blocks out WishBliss's only attacking move from doing much. Miltank needs Heal Bell to cure its Toxic. :x Body Slam Miltank is a 4HKO, which would, again, end up being a stall fest with Leftovers + Wish + Softboiled. @_@
As for the individual Pokemon, Blissey comes out on top. It's OU, compared to Miltank's being UU. Blissey is defensively one of the best Pokemon in the game.