My single console universe is showing. Sorry other consoles. You were either too little too late, or just too little.
1. Call of Duty 4
2. Crackdown
3. Portal
4. Bioshock
5. Halo 3
6. Mass Effect
From least flawed/most replayable/most enjoyable to least. COD4 is just flawless for what it is. Crackdown is just simple fun, is replayable as hell, and has a lot of little things to do to keep you busy for ages. Portal was awesomeness, but short (yeah, yeah, I know), and I didn't really feel the replay value. Bioshock does almost nothing wrong per se - but the story and gameplay flatlines after about midway through. Halo 3 was a technical achievement on many levels and true fan service, but the jaggy graphics (fucking atrocious in some areas), horrific cinematics/exposition, and weak deployable system keep it from being a true classic. It still the best Halo game for pure gameplay and set piece battles though. Mass Effect has *many* flaws, from purely technical ones like pop-in and load times, to bad/wonky UI, lack of explanations on actual mechanics, and the dullest sidequests I've ever seen (except for maybe Assassin's Creed). But what is there grows on you, and the small innovations they made to their own damn system tied to a decent story make it more than worthwhile.