Ghost recon is a realism based combat special operations game. The graphics and movement in this game simulate a real enviroment which can be very tough.. also very realistic baddies as many times unless you are very careful you won't see them in the brush and trees.
Counterstrike is Team based capture the flag mostly.
Wolfenstein is a 3d shooter with some very cool objective based teamplay somewhat like CS I suppose.
capture the flag(s) set the bomb etc..
I honestly like all three depending on my mood hehe.
There are some addons for GR and a whole new game sum of all fears built on it's engine. I imagine they are all worthwhile.