Ok, I'm gonna be captain obvious on this one, but Halo (all iterations there of except Halo Wars, which I have not played and therefore cannot comment on). Solid game design? Sure. Good brainless fun? Absolutely. Great game to have on the shelf when you've got mates around? Fucking A. But seriously, overall its average. Not good, not bad, just average with an above average level of polish.
The call of duty games are also guilty of this. Very highly polished mediocrity is not brilliance, no matter how much elbow grease you put into working the shine out of it. Don't get me wrong - both those game series have a lot going for them - but its all on the surface, bite in and there's very little there.
On a slightly less obvious and more controversial note, the original starcraft. Was never really that impressed. Still haven't got around to playing starcraft 2.
Oh, and slightly off topic because its supposed to be games you ENJOY and think are over rated, but my number one over rated game of all time has to be counterstrike. Seriously. The only conceivable use for it is as a coaster, and you can't even do that because its digitally distributed not on a CD.