I actually think Halo is very underrated. For every one person who likes it there are at least two who think it's crap only because it started out on a console made by Microsoft.

Anyway, Grand Theft Auto, XenoSUCKga, Final Fantasy VI, VII, VIII, IX, X and X-2, World of Warcraft, Soul Calibur, Final Fantasy Tactics, Ocerina of Time, and every racing game ever made qualifies as overrated.