Depends on your style.
If you like graphics over gameplay mostly, you will like "Crysis".
Anyway, my list consists of gameplay on top over graphics!
1.
Gunz - Gunz is a free tactical TPS game. Though, you might see a lot of hackers after 3-4 days of an update. It stays a fun game to play and only takes around 400-500mb? Game focuses a lot on skill but also on your level in-game
Link: http://gunz.ijji.com/2.
The elder scrolls IV; Oblivion Oblivion is a RPG. It's pretty deep for a 3d RPG. Might check it out :P I recommend using FCOM mod as it balances the whole game but the game get three times as hard
Link to mod: http://devnull.sweetdanger.net/convergence.html ( kinda confusing if you never done it before )
Link to homepage: http://www.elderscrolls.com/games/oblivion_overview.htm3.
Warcraft III Despite it's old, it's multiplayer is very good. You can choose: normal competition ( mostly pros ) or custom games. If you choose custom game you can pretty much do every game mode you can imagine... are you a LOTR hero who has to defend the fortress? Or are you the evil mordor forces to attack the fortress and destroy it. Or perhaps you are building towers and stopping waves of enemies...
Link to homepage: http://www.blizzard.com/us/war3/Other: Game sucks without the expansion The Frozen Throne
4.
Mount & Blade Ever wanted just to be in a big world where you can travel everywhere you want and control your own army ? well this is your chance! with this TPS ( medieval weapons and stuff ) you can slaughter your enemies together with your own forces.
I recommend trying the trial to see if you like it. Its MODS make the gameplay even bigger! want to be in a LOTR enviroment just install a MOD or want gunpowder weapons install a MOD ! very easy.
Link to homepage: http://www.taleworlds.com/