You're right in saying Micheal wasn't really anything - just a guy with a high pain tolerance and a mask of Captain Kirk turned inside out and painted white (look at it and imagine it... I'm telling you, it is.)

As for the previously mentioned Invincible guys - Practically every horror film nasty's invincible. They killed Freddy a thousand times over, but he just kept on coming back. Jaason didn't die until the fourth one, I think, but then when he got made into a zombie-creature in six, that's when the immortality thing kicked in. Micheal's already been covered above.

Pinhead is an immortal: Even though he dies, he does come back because of the magnitutde of his evil, even though Kirsty makes him remember breifly. Candyman kinda pulls a fuzzy memory at the moment. The only reason these things are so damn unkillable is more than likely so they can make a sequel.

In terms of soaking up damage and keep going, I think the main guy here is Ash. We know he takes the beatings, we know he gets cut, bitten, self-mutilated, stabbed, almost drowned... he's either got an incredible tolerance to pain, or he's ust too dumb to realise he should have diead in the first one. I dunno, mybe one day it will all catch up with him oneday when he's working in S-Mart, and he'll just kinda implode and flop to the floor.
What kind of world are we living in, where a man dressed as a bat gets my pulicity?