These are in my opinion the top 15 multiplayer FPS action games at the moment:
1) Call of Duty 4: Modern Warfare
2) Battlefield: Bad Company 2
3) Call of Duty 2
4) Call of Duty: Modern Warfare 2
5) Counter-Strike: Source
6) Battlefield 2
7) Call of Duty
8) Wolfenstein: Enemy Territory
9) Team Fortress 2
10) Medal of Honor Allied Assault
11) Call of Duty: World At War
14) Combat Arms
15) Half-Life 2 (All Mods)
These are the most played shooter games at the moment, therefore the most successful at this moment. Call of Duty 4: Modern Warfare has only been overtaken by Modern Warfare 2, but this happened only for 1 or 2 days, when Modern Warfare 2 came out. MW 2 is a very fun game, with a lot of diversity and even though it's not as balanced as Call of Duty 4, it is definitely a very fun to play game.
At the moment, Call of Duty 4: Modern Warfare is the most played game on PC after World Of Warcraft. The same was Counter Strike 1.6 when it came out and it dominated the market for several years. Then, Counter Strike Source came out, basically the same game with improved graphics. Also, Counter Strike Source was the last game to hold the number 1 spot as the most played video game on PC before World Of Warcraft came out.
I think that if Valve would decide to release a new Counter Strike, it might become a "threat" to World Of Warcraft domination and it will take the number 1 spot as the most played fps or shooter game.
Another shooter that came out and quickly grabbed on of the top 3 spots is Battlefield: Bad Company 2. How long is it going to last up there? We'll have to wait and see that.
The Call of Duty series have been very successful. Call of Duty and Call of Duty 2 were the ones to open the road to success for the following games. With its innovative game play, the ability to aim down the sight and the feeling you get while playing it, being in the middle of World War II, these 2 games have made history in the first person shooter games.
Wolfenstein is one of the greatest shooters there. A very original story with Nazi zombies as well as an interactive game play with a lot of visual effects, this game has made it into the top 15 most played fps games.
Crossfire and Combat Arms another 2 great fps games, each of them with a different story and a different set play. Different weapons, maps, game play but all of these create a great environment in both of these first person shooters.
In conclusion, I think all these games are great, each of them has something special and different to offer.