Oof, these are some tough descisions…
Lets see…
FPS: TF2, Natural Selection (Link:Natural Selection, its old but its fun if you have HL), Bioshock
Strategy: Red Alert 2, Warcraft 3
RPG: Not a fan of RPG’s really…
MMORPG: City of Heroes/Villains (A very refreshing change of pace from regular MMOs), WoW i guess if you dont have to pay
Sandbox: Garry’s Mod 
Send me a PM for more info on NS