eh, i dont know if you wanna play Half life 1. you will be MUCH better off playing half life Source....cause heh....half life looks like complete shit right now. I booted it up just for nostalgia the other night and wow, its so damn ugly compared to how its probably gonna look in source.
and yea you should play both halo and halo 2. And with Halo 1, you should play Co-Op as much as possible cause thats where Halo 1 shines alot imo.
halo 2 shines in both co-op, single, and online.
peace