I rank endings based on how well they tie things up and the satisfaction they give for reaching them. I don't count cliffhangers, those aren't endings, just insurance on the part of the developers to guarantee the sequel they want to make.
I would've liked Crackdown's more, but I had kinda expected it because of his attitude towards civilians.
I really liked COD4's, because of how well it conveyed just how brutal modern warfare is (or how brutal it seems like it is).
Mass Effect's ending is just pure awesomeness. It does lose points for being a partial cliffhanger, but it tied up the issues specific to the first game of the trilogy (Saren, the initial Reaper invasion, and Sovereign), and had truly awesome credits music. Seriously, it felt like the perfect payoff for playing the game to completion.
On that note, Portal's ending was/is genius. GlaDOS has great vocal range.
Halo 3 has the best final level I can think of off the top of my head. Admittedly, the first half is mediocre Flood combat, but the epic driving after that (and finally getting to kill that damn lightbulb) set to the "epic" version of the Halo theme, is gaming nirvana.
Oh, and Mirror's Edge was pretty good too. Mostly because you got to listen to the whole theme song for the first (and only) time in the game.