My favorites(not in any order favored order, just some likeness of genre order):
(Halo talk after)
Half-life
-mods: Someplace else, they hunger, Minimicus, timefall, TFC, CS (at times), PQL, and issues.
Half-life 2
-mods: Minerva, Iron Grip, and im putting Black Mesa on this list even though its not public yet.
One of the reasons I love Half-life is the mods. the main games are really great on their own, but between the two of them and their mods i've gotten more then 200 hours of Quality gameplay.
Halo 1: like Reno said, the shield system made the game fun, and the weapons were somewhat unique.
Doom and Doom II: First things I mapped for back in 98, very fond memories
Marathon
Steel Batalion: its amazing what a massive 50 button custom controler does for an Xbox game with ok everthing else
Battlefield 1942
Goldeneye 64
Call of Duty 2
Metal of honor allied assault
Total Annihilation: The greatest RTS ever
SD Gundam X: Super Gatchapon World: Second best ever in my opinion. its a SNES game relesed only in japan, after seeing my friend play it I got it and spent an hour fiddleing to figureout what menu option does what. and I still play it to this day.
No Greater Glory: feels like your playing in 400x300 its so old. and I have yet to beat the CPU even on ultra easy
Rise of nations
C&C Tiberian sun
C&C Red alert 1&2
StarCraft: we all know it and love it
Warcraft: another blizzard classic
Populous The beggining: addicting
Age of Empires 1&2: not 3 tho, 3 was just bad
X-Com: and most of the series
Baulders gate 2
Arcanum: first RPG i ever played, still fun to play again
Blade of darkness: got it on the same demo CD as Arcanum came on, getting both was a wise investment
Betrayal at Krondor: and the rest of that series, it was just so darn addicting despite horrible graphics
Neverwinter knights
Dungon siege
Diablo series: <3
Fallout series: even bigger <3
KOTOR
Zelda Links awakening DX
Zelda A link to the past
Zelda Ocarania of Time
Deus ex
Falcon 4.0 allied forces: one of the few flight simulators that was fun.
Descent: awesome fun for hours upon end
Super Smash Brothers
Super Mario 64
Super Mario Cart
Super Mario World
Metal Gear and Metal Gear Solid
Final Fantasy 4-8
Chrono trigger
NoX
Lemmings
Asteroids
Tempest
Torus trooper
Twisted Metal
Super Mario Bros 2
Missle command
Hydro Thunder
Time Crisis 3
Shadow of the Colossus
and a few others I know I missed
Now on to Halo
*Halo2 Is the Most Played Xbox online game
this quote has been bugging me the entire time i've been writing this post. it doesn't say anything about halo, except that its owned by a high % of people who play Xbox. That doesn't make it a truely good game, since more people play CS and CS:S then have Xbox live accounts.
I still play halo sometimes, its fun, especially on legendary because its a challange. Halo was fun because of the shield which protected your health. Halo 2 runined that by taking out the health, so your shield was the only thing you had to go by. having health really added consequences to you messing up in Halo 1.
IMHO marathon was a better game. The plot was better developed. as opposed to Halo, which, while it had a good way of advancing the plot in Halo 1, dropped alot when halo 2 came around.
and I've done alot of digging about Halo 3. there aren't alot of secrets left for me to find out. it won't be much of a surprise to me anymore.
Ill tell you this about Halo 3, if you thought that anything about the other two was hit or miss, then halo 3 will blow you away or fizzle out into nothing. Bungie is all in on it.
and if you want an idea, think of putting Halo 1 and Marathon in a super colider with a dash of Half-life and Shadow of the colossus, and you have Halo 3 after some interactions that could destroy the entire thing.