Well, half-life 1 and 2 are great game, I totally agree, but not best in fps.. Well, I don't have played lot of different fps, but I would say halo serie are really good one, counter strike is a great too, but now less because theres some better games that came out... but when it first came out, it was best fps ever Well, Im not up date with the new fps, so I can't talk about the really recent ones..
I'd say it's really all up to opinion. But the majority will say Halo or COD because they haven't played anything else and if they have it was a long time ago and haven't played anything else.
I've played X-Box and I didn't like Halo. It's like an FPS for wannabe gamers who suck at REAL FPS games. Socom is my favorite... even though it's not FPS... I play it in FPS mode... but you actually have to THINK for that game, so most people today couldn't play it.