So, what are they, and why?

For me it would be the following:

Half life 2/Gmod: What can I say that hasn't been said before...simply amazing for a 2004 game, and if you haven't played it yet...go buy the orange box! Also, Gmod is the most amazing add-on ever for any video game! A must have if you enjoy playing with physics and building stuff =D. In our ID tech camp class last year, our teacher used this game as an example of a amazing storylined game with emotion and "movie-like" style.

Left 4 Dead: Awesome co-op zombie killing fun, always a blast to play with friends online, and it still manages to run on a cheap $400 Acer laptop (well...it did have a X1250 IGP in it to help it... =P)

Crysis: Best. Graphics. EVER (Of course if you've been hearing about it, you already know this). Gameplay, although not the best, is still enjoyable and fun. The editior that came packed with it is so amazingly easy I was able to build a huge photo realistic map in about 20 minuets (minus scripts and AI coding...).

The Elder Scrolls 4: Oblivion: One of the best RPG's ever, and I still play it from time to time.

Call of Duty 4: Best FPS for online play so far, In me and my friends opinions, Call of Duty World at War failed miserably, weapons were totally unbalanced in multilayer, and you could have a much more enjoyable experience on Call of Duty 2 if you wanted WW2 action.

Metal Gear Solid 4: Seriously, this is better than most movies that I've watched. Buy this game, and get some popcorn while your at it. All scenes give the sense of emotion and lively hood to Snake and company, and the ending is almost tear-jerking if you've been following most of the plot of the Metal Gear series, and even if you haven't, its still amazingly emotional.

Man, I love Half-Life. The graphics ARE amazing for a 2004 game. Halo 3 is fun too though. Of course, I love Armagetron as well because of its addictive open-source nature.