The world of Diablo has many monster types, and their diversity is one to behold. But one type stands out from the rest: the Superuniques. These guys are the baddest of the bad. The ones you need a group to kill, or at least a good combination of both skill and hand-eye coordination.
One of the first we have seen (which might not be a Superunique, despite his awesomeness) is the Siegebreaker Assault Beast. This nasty monstrosity has size and strength on his side. Brute force and imprecision. When he kills a player he does a random fatality animation, seen first as the Barbarian gets his head bitten off. A most memorable image. The death animation of the Siegebreaker is also special. Instead of dying in a big blob of gore, his corpse will turn to stone and crumble opening up portals from hell around his remains.
Then there is the Thousand Pounder that cultists would summon. Granted when he is summoned he comes in with an explosion knocking back and possibly killing those that summoned him. He also enrages when his health is low and begins to glow and start attacking faster and does more damage.
Finally we have the famed Skeleton King. This boss was seen during and at the end of the Wizard reveal trailer during BlizzCon 2009. Leoric is first seen as a ghost taunting the wizard every step of the way in the dungeon. At the end of the reveal video his ghost returned into his skeletal body getting up and carries a massive maul as he walks towards the wizard at the end. A nice comeback of a classic Diablo I boss.