Its so hard to choose, but I choose Halo for its originality of making all of those allien races, and especially for the multiplayer. Half Life is a good one too, but its more realistic to see an alien that a strange monster...
Both are on completely different calibers of gaming. If you're talking story line well I personally like the Half-life story better. gameplay I can't really say... Half-life has got steam behind it and Halo has got... Bungie? ummm yeah... Q(-_-Q)
HALF-LIFE!!!! I love playing through Ravenholm, running over zombies, and stuff like that. Better graphics. And WTF?!?!?!? Master Chief would own Gordon Freeman?!?!?!?!??! NO! HELL NO!!!!! MC can just kiss his @$$ bye-bye when he sees Gordon Freeman sprinting at him with his crowbar!