Fallout: New Vegas > Halo Reach > CodMW2
^^ this is what I believe
I know I'm super late in saying this but I just wanted to make a comment on this.
I personally believe... That CoD MW2, Fallout New Vegas and Halo Reach are too different of games to really be compared to each other like that. Pretty much the only thing that ties them together is that they are all FPS games.
#1 Fallout is an RPG. You can affect how strong your character is by leveling up different skill attributes. You do quests and side missions and pretty much have free reign over how you want the story to play out.
#2 Halo Reach is more of a classic type shooter in the fact that you cannot use iron sights and the bullets use hitscan. The campaign, while being epic in its own way, will be linear as well. It borrows a lot of ideas from other games (loadouts, not like MW2 create a class though) but it is still Halo.
#3 Modern Warfare 2 is a very progressive, modern shooter. It, like Halo and a lot of other FPS games, has a linear campaign. The multiplayer borrows from RPGs a little by giving you level advancement and rewards for doing certain things in the game (Halo Reach does this as well). MW2, unlike Halo Reach, has killstreak and deathstreak awards, titles, and no vehicle warfare.
I just wanted to point those things out. Not trying to put you down or anything!
(tl;dr version - Fallout is an RPG. Halo is a fantasy shooter. MW2 is a modern shooter. There ya go.)