Im gunna go on a giant rant here on my fave different genres and titles out there...
FPS:
L4D was great at first, now its lost a bit of its charm (origonally it was scary as hell) and a bit of a bore tbh.
TF2 is hilarious fun, and one of the most balanced FPS games ever made, seriously. And all the free patch updates and class updates that valve keep doing is great for replayability
COD4 revolutionised the serious fps online, and its not been beaten so far imo (WaW is CRAP) MW2 is highly anticipated...
Unreal Tournament series. Crazy fast and a whole bunch of fun unique weaponry to explode people with. A massive shame that so little people play UT3...
The best however is the old school Quake 2 and its mod, Action Quake 2. Ridiculous physics made it even more fun (jumping around the map at about 30 mph while holding a scoped sniper rifle?)and great ideas such as area specific hitboxes(leg shot cripples you until u bandage, stomach shots will pretty much leave u with no health left and a headshot will....explode you haha), and its gameplay being faster than any other FPS EVER. its a shame the game is pretty much dead now what with being incredibly old.
RTS:
C&C series has always been the hallmark of RTS games, with addictive styles and hilarious storylines/acting. The latest offering RA3 is nothing new per se, but great fun nonetheless. I still prefer 2/yuris revenge tho.
Warcraft series, before WoW and subsequent releases. They were fun, different and really quite addictive (the hilarious vocals for units selected in game) and the storylines/cinematics were really impressive. That said it IS now purely focused on the MMO side...
AoE2. Endless hours spent as a young teen LANing with this great game, cheats or no...
Theme Hospital. One of the best games ever. EVER. I would fully reccomend every person no matter the age gender or proffession to give this game a try. Even as old as it is I think its unique and hilariously fun and addictive. My only gripe is how ive gotten so close to completing it several times and had my save file get corrupted....
Dungeon Keeper. An old favourite of mine ive spent possibly 100+hours as a kid and teen playing this game. Blends innovative design and simple gameplay with suprisingly indepth micromanagement and heavy use of tactics. Not to mention being sadisticly hard(i still have never completed more than 2 or 3 of the expansion pack Deeper Dungeons). Being bad is good....shame DK2 was kinda crap in comparison.
Im gunna stop now cos I feel like playing loads of games and I have work to do....
TL;DR much?