I haven't got to play CS: Source and Condition Zero, so I can't compare them. They seem to be good, though: I've heard Condition Zero has a singleplayer "story mode" of sorts, and that makes it sound pretty interesting and present somehow an enhanced replay value. On the other hand, with the usage of the source engine in CS:S, it seems like it's a revamped version of the good old CS 1.6 and, well, CS: 1.6 was awesome enough; CS:S has everything to make it more awesome.
But as I said previously, I haven't got to play those, so I'll compare the ones I got to play: CS: 1.6 and CS:GO
CS: 1.6 is the best for me. It's light, has average graphics, tons of stages and that nostalgic feel of playing it in LAN mode with many friends. I still get to play it nowadays. It's probably the most important FPS of the genre.
On the other hand, I'd place CS:GO on the last place of my list. It's pretty, I know. It has many kinds of weapons, I know. But it doesn't react like a genuine CS game. I mean, it has a whole different atmosphere and the weapons don't sound as appealing as they were before. And, even though it'd make it sound like a Call of Duty game, if they were wanting to make it the best CS of all times, they could, perhaps, add iron sights and attachments, right? Making it a CS: 1.6 with improved graphics and adding random guns make it seem like they didn't put any effort in that.
