I consider singleplayer FPSs to be in a different category than multiplayer. The gameplay of a game is changed a lot by other people. Some games would be a lot better with a good community, and some games would be really bad without the community.
Also, please note I have never owned a single console, except for the Wii, and I have yet to play it. Though I have played some games. Also note that these aren't really in any order.
Anyway, HL2 is a favorite for everyone, I really liked the episodes too, mostly Ep2, though that thing with the guardian was confusing, I really had no idea where I was supposed to go. The final thing was rather difficult as well.
TF2 has to be mentioned again. I don't like Halo,
I haven't actually played very many TBS games other than the Civ series. Other than Alpha Centauri, which quite literally was Civ IN SPAAAAAAAACE. I really liked the government choices in that though, glad they brought that style back in Civ 4.
Black and White. Has nobody mentioned this yet? This is an amazing game, and I wasted so much time playing it. I don't have the second, but I've heard it isn't as good. I once had an amusing glitch where I put a pregnant women into the ocean, I expected her to drown of course. Instead, she gave birth, and when I went back to look at her later, she and her child were walking on the ocean. I don't really run into glitches, ever. I haven't ever seen a single glitch in a game except for that one, and a couple of times in various games. Nothing gamebreaking though.
I really enjoyed Mount & Blade, or at least the demo for it.
I don't really like many RTS games, because of the consistent focus on micro. I personally hate microing, and am no good at it. I like TBS a lot better. If anybody knows some RTSs that don't have microing, or at least minimalize (is that even a real word?) it, I'd be glad to try them. Too many RTS games focus on actions per minute, and I really hate it.
I really like RPGs, though I want more. Oblivion was rather enjoyable, though like other people have said, it's only really good with mods. I really want to know people's opinions on Dragon Age: Origins.
If RPGs aren't my favorite genre, then sandboxes are. Garry's Mod is amazing, and is just that much better with Wire. Blockland is a pretty good one too, though the community can be bad sometimes. Also, Vertigon is wrong about "a couple hundred". Try a couple hundred thousand. I think the actual limit is around 256,000. Absolutely insane. The modding and events are really great too.