Quake Wars, Crysis, TF2, UT3
You MUST consider system specs. The half life engine was runable on almost everything- so CS became the god. Same with Quake, engines that optimized what people had, and produced a fun game.
Crysis will be at the bottom of the stack because its an eye candy whore of a game
Quake Wars seems flat...fun but flat for some reason...not sure if the masses wil latch onto this like a CS title.
UT3-Huge following already and you can bet on having it working on lower end systems with great results-already a cult following of this series and it will continue
TF2-this will sweep the above, because, the source engine is very efficient and runs like a knife through butter on most if not all systems set up for a gaming experience. I know the CS S ran great on a laptop sporting a radeon 7500...so this is an efficient engine and its a fun game with great balance and simplicity, but yet complex enough to keep people coming back!