The best shooter is BF2 mod: Project Reality imo.
I have yet to find a more challenging, realistic shooter.
and it's free! (the mod)
(bf2 costs like 9 bucks now, or get from the bay of pirates)
The mod says it all, it's a mod to make the most realistic shooter ever, and imo they succeeded over any other game.
Very tough, and servers are always full.
http://www.realitymod.com/
Should check it out!!