Best "pure" (primarily) SP shooter in recent times, for me, was Crysis. It was just amazingly beautiful and had a decent mixture of freeform versus directed gameplay. I think it's the only shooter I've played all the way through since that Wolfenstein game before the last one. Return to Castle Wolfenstein? I think so.
Crysis 2? Zzzz.
Far Cry was very pretty for its time, but it didn't do much for me. It was altogether too samey.
Far Cry 2 was more interesting with its setting and the non-linear approach, but the checkpoint design killed it for me, especially with those guard camps with respawning.
One game I consider really underrated was Unreal 2. No, it wasn't that great - and yes, it was rather short - but it had some really cool levels and a strong atmosphere. I really liked it, anyway.
I miss a REAL Unreal sequel. As in, not just the engine - and not just a MP Tournament game.
The original Unreal remains one of the most impressive games of my life, in terms of when it was released and what it did technically. I think it went beyond Doom and Quake for me, personally.
Others? Hmm… So many. Dunno. I hate modern military SP campaigns. The same shit over and over. I kinda liked that semi-Bioshock clone from a year back or so. What was it called again? The latest Wolfenstein (also by Raven) was middling at best.