Specifically single-player? I'll go with Halo - I never really 'got' half life. All the revolutionary charm people claim it has basically seems to come down to 'it has physics puzzles'. I like the no cutscene approach to story-telling, sure, but the rest seems kinda generic. Also, I prefer regenerating health/shields to medpacks, and hate the point where you have to creep through each room agonisingly slowly because you have 1 HP left and have to clear huge rooms of enemies with it.
Recently played through the first Halo game courtesy of the anniversary edition, and loved every non-library second of it. Hard-pressed to say why, admittedly. I also find the series in general to have a greater replay factor than half life or CoD.
CoD's singleplayer has gone a long way downhill since the first modern warfare, and is now merely an add-on to the multiplayer. They should stop including it now the MW story's all wrapped up.
Since I'm here, let's go off-topic onto multiplayer!
Half life doesn't have any. That's not inherently a bad thing, as long as the story has the length and or low price to back that up, which it does here.
CoD does, and it's highly successful. What I can't work out is why, since no-one enjoys it whilst they're playing. The adrenaline levels are so high that very primal rage responses come out, and everyone finishes a session incredibly angry (the sample size for 'everyone' here consists of everyone I've seen play it. Even me, and I'm usually unaware of what anger even is)
Halo also does. I'm a huge fan of the series, so providing a remotely objective view of it will be nigh-impossible. It doesn't have the tension of the CoD (or any other 'realistic') shooter, so seems to induce slightly less aggression, whilst offering broadly similar gameplay (by which I mean pull the trigger until the other guy falls over).