sauvignon1: I've always felt Half Life II was overrated. Yes, the technology was/is impressive, but the story feels rather weak and the gunplay is just ok.
What bothers me most is the complete lack of originality. Half-Life 2 is an incredibly soulless game, IMHO. There's generic totalitarian stuff, generic sci-fi stuff, generic horror stuff and a really dull plot with really uninspired and unbelievable characters. The most original thing was that Eastern European design of City 17 but heck, what's up with that? Why? Just why? Sure, the character animations and physics were impressive - but all in all the game felt like a tech demo, especially in retrospect. Half-Life 1 was also kinda generic scifi + conspiracy stuff but to me it always felt like a parody with lots of black humor and clever homages which kinda justified the whole design. I like Half-Life 2 but it's not nearly as good and sophisticated as people make it appear.
Anyway: my ultimate overrated game is Empire Earth. IMHO it's just crap. Real crap. Crap crap - crap crap crap - crap. The whole game is one big design flaw and as if that weren't enough even its technical execution is just horrible (how the modern warships got past QA is a riddle to me).
One of my ultimate underrated games is Vietcong. To me it's one of the best war-themed games ever created (only talking about the first one, mind you - not the botched sequel). That unpredictable AI haunts me in my dreams to this day. Plus it had lots of novel ideas which were even executed extremely well and it really felt like the developers had done a lot of research to capture the atmosphere of that particular conflict.