When it comes to Halo, I loved the vehicle designs. They had a real world look to them as practical machines. The Warthog, drop ship (obvious influence from the Aliens film), and then the alien craft with an completely unreal but still practical design.
I still play the original on my Mac in crazy high resolution. It may not be as pretty as COD4 on my setup, but 2650x1440 makes sniping almost too easy in multiplayer.