Fedor
After losing to Werdum, BF, and Hendo, he came back to whoop on Monson, Ishii, and Pedro Rizzo.
That's not really a comeback. Especially seeing as he quit, he wasn't facing contenders anymore.
Werdum!
Couture. Maybe more than one even...
Don't teally count him as having a 'comeback', Wedum was always a top HW and relevant to the division as a whole, he just had an unfortunate career hiccup after losing to JDS and getting screwed by the UFC.
Belfort and Brown are the top for mine, and I think Lawler is up there too. Hopefully BJ Penn is a shocking comeback in the making!