Half-life 2.
I've never really got into Halo because the environments in the early ones are very identical and there's no real stark changes.
The Half-life's campaigns had huge changes, from the interiors of the labs, to Xen, to coastline, city apartments, the citadel, sewers, sandtraps... there was so much change and as a level designer I love picking through the lighting, shapes and general constructs of environment that made up the entire thing.
Halo just has a lot of identical corridors copy-pasted together, with the occasional change to top of world grassland and tundra. There were also the interior spaces, but navigation through lighting and intuitive mechanisms are scarce. Not to mention the huge cliché's used in the script.