mohaa is better in my opinion because of graphics and gameplay...
but half life (specifically counter-strike) has alot more guns and i love how you can basically customize your setup, like you can buy certain guns and pistols, armor, and nades
I like the Alien vs. Predator 2 MP game. All three species have very different advantages and disadvantages and it really brings out strategy in the game.
The multiplayer isnt that fun..the Mp5 looks like SHIT and for me thats a drawback for SOME reason..
Also the single player scared the buhjeezus outa me. I hate the flying chickens and zombies and not knowing what the hell is going on!!
Ill have to go with saying Counter Strike is my fav. Me and my freinds from school started a clan and we just mess around in our server..it isnt our lives though