Halo 2 was the xbox game that everyone with an xbox had, cause there wasn't much else. Indeed the far more successful PS2's best selling game San Andreas sold twice as much.
CoD 2 and 4 were both good shooters, but I'll throw my hat in for Modern Warfare for the same reasons I stated before. The game established a sense of mortality that I think more game developers should try to work with. Even Treyarch can't seem to get their heads around the concept.
If we are arguing for redemption I'm going to suggest Diablo 3 again. It's new, and many people are still blinded by nostalgia for a 12 year old rpg. If you want just the best games on your list, you want Diablo 3. Freeing up resource systems, giving players more than two different abilities to use at once, getting rid of basic attacks, the absolute variety of 125 abilities per character before passives, the fact that all abilities scale, the lack of dead levels, the introduction of cooldowns and other systems to encourage diversity (generator/spenders), that is 7 things that every Diablo clone should have from now on. Diablo 3 is one of the biggest improvements as a sequel ever, and should be seen as a genre changer.
Dragon Age: Origins. Who the fuck voted no? Ok, I'm not Bioware's best friend at the moment but Dragon Age was the best Bioware game, and even really the best d&d esque fantasy game ever made. This is another prime role model for RPGs (one that the above Diablo 3 may have taken things from), requiring a great amount of strategy. You could script your allies and use a variety of abilities diversified by cooldowns. The mechanic for active abilities was genius. Each class had a variety of builds you could use, and damn that mage. Dragon Age is THE video game for playing mages. Then there is the surprisingly fresh story and always great characters. The ending even delivered. Remember when endings could just be good and show you what the effects of your actions were? Good times.