The Halo games are overrated, very good, but they really are bigger than themselves at this point. However they aren't the most overrated games ever. That title goes to Final Fantasy VII. Gameplay is alright, graphics were good for their time (not great), story is weak as far as FF games go, the ending is a terrible anticlimax, and if it weren't for the marketing blitz behind it, it wouldn't be as highly regarded.
All the FF games for the PS1 qualify for great games I don't care for. 10 and 12 on the PS2 were great improvements, but still not up to Final Fantasy 6 in my opinion.
World of Warcraft I don't care for, as it is another game that's grown bigger than itself. Its a good MMO, but I don't like MMOs. MMOs destroy their surrounding fiction by bringing a bunch of idiots in to populate the world. You can't have a real story with 8 million people playing, its too unwieldy. You can either have the players all be heroes instrumental in the storyline which defeats the illusion, a'la Guild Wars, or you can have the players be entirely inconsequential, and the storyline goes on despite them, not because of them. that defeats the whole point of games.
Metal Gear Solid series, I've said it many times, Hideo Kojima missed his calling. He should be making movies, not games. And right now the line is pretty blurry, there's that many cutscenes.