Day of Defeat, Day of Defeat: Source, Battlefield 1942, Call of Duty, Call of Duty: United Offensive, Call of Duty 2, Brothers in Arms: Road to Hill 30 and Brothers in Arms: Earned in Blood.
Personally I hate the Medal of Honor series...they were okay, but the gameplay was nothing compared to Call of Duty or Brothers in Arms. I would recommend Day of Defeat: Source as an alternative to Call of Duty's online multiplayer. The Source engine really compliments the gameplay well.
Battlefield 2 would be my first choice, followed by Call of Duty 2.
Although, the problem with online gaming these days are the cheats. I have never worked out the point in cheating. The gaming community have tried to stop cheating with PunkBuster, but many cheats are not detected for example on BF2 you have Clear View,Tank Jacket, AAbot, Clipper, Radar001 and others that get past the PunkBuster anti cheat software.
In the end, it's down to the individual to enjoy gaming and don't take it too seriously.
Yeah, Battlefield 2142 looks cool. Lets just hope and pray that DICE doesn't screw up and make it a bugtacular game like Battlefield 2. I swear, if Patch v1.4 is just as buggy and annoying as v1.3, I'm gonna send a hate letter to DICE. I've had enough with all the crashes and long-ass loading times.