Call of Duty (Standard) - very engaging and action-packed, very fun
Day of Defeat 2.0 - very enagaging and actioned-packed, very fun
Call of Duty (Heat of Battle Mod) - never tried it
Far Cry - never tried it
Rainbow Six Series - ultra-tactifcal, requires thought and skill, not as action-packed but very fun nonetheless, if not more fun that shooters
Counter Strike - a timeless classic, small learning curve, yet deep enough to satisfy, requirs much skill, very very fun
BattleField 1942 - arcadey, very well rounded, a good server is required for optimum enjoyment, very very fun
Battlefield Vietnam - require more skill and teamwork than 1942, harder learning curve, requires patience but very very rewarding and fun
Blackhawk Down - bad game, too easy. Should never have been made, as it is very disrespectful. Not something to make a game out of.
America's Army - requires a lot of skill, hard learning curve, very tactful and realistic, very fun, geared towards those who do not just want to run and gun. very fun
Operation Flashpoint - never played it, but a very good game
Hidden & Dangerous Series - requires much tact and skill and decision-making, hard learning curve, fun
Medal of Honor Allied Assault - Rambo-ish, not too much skill required, very action packed and run and gun, very very fun
Medal of Honor Allied Assualt: Spearhead - Rambo-ish, nice improvements upon the first, too short, very fun
Medal of Honor: Breakthrough - by this time the game has gotten old. Very little new content. Not worth it, and too short on top of it.
My estimation of things. :rambo: