If we're talking SP campaign, then I think Half Life 2 probably represents the best FPS SP campaign yet that I've played.
I see people talking about Unreal Tournament and Quake and those were great too, but not as SP games. They were all about the Online MP, which is not the context of the OP so far as I can tell.
HL2's narrative was nicely done, and surprisingly subtle in its use of level design to convey information about the Earth's plight with lowered water levels, etc. It also boasted what remains some of the tightest, best FPS shooting action ever coupled with a constantly evolving scenario that saw you pilot boats, drive cars, order around your own 'pet' antlions and solve puzzles. The careful flow of the levels also took you through everything from futuristic thriller to horror survival.
Even today (and I played it again just recently) on a high end rig with everything maxed and hooked up to a big HD TV is still looks and plays better than anything else I've tried.
I've still to play Gears 2 (although I've played number 1). And while elements of Gears come close to HL2 (and Half Life before it) the gap to me was still pretty big, and I doubt Gears 2 is going to be that much closer (but I'll give it a go of course, it will be a lot of fun there's no doubt about that).
Try to be reasonable... its easier than you think...
