Even though Dark Souls has it bosses, I'm going to throwback and go to Gears of War.
Favorite boss, most definitely Skorge. Fighting Skorge was pretty fun, contrary to most bosses (DARK SOULS PLS). He bounced around, was actually pretty hard on insane if you found him alone, and honestly he was my favorite boss in the whole game. Not to mention he was a total badass. I mean, come on. Chainsaws on a staff.
Most disappointed, as Hyde went, is fucking Capra Demon. What sort of shitty Side Boss man. The fighting area was so damn small, and the hounds that came with him didn't help. I honestly just wanted to kill myself when I fought him the first few times.