I originally tried to play SoC vanilla but didn't get very far. I installed Stalker Complete for that and loved the game. I think CoP is the better of the two and I played CoP without any mods.
I've read this one is quite problematic. Will Complete clean it up and make it playable? Is it even worth trying without any mods? One of the things that bothered me the most about vanilla SoC was the horrible swaying and constant knife animations while walking/running. As I said, I didn't get far without any mods so I don't even know what the original gameplay is like. --- Drink your milk!
SoC is the better game, but you're entitled to your opinion (even if it's wrong ;p). CoP is still a very good game though
Clear Sky is by far the weaker game of the trilogy. you will need the latest patch (1.5.10) otherwise it is pretty unplayable. the game is fine without mods until you reach the ending where the game really goes downhill, but it's a very long game so... the ending is really disappointing though, so don't expect much from the ending and you'll be fine. the maps are mostly the same as those from SoC so it will feel very familiar, though as it's a prequel there are a few differences. the gun upgrades are a welcome addition.
I didn't really mind the stories in the other two games. They were there but nothing amazing. The game itself is what keeps me playing. Stalker is truly unique and I've never played anything like it.
I played CoP first so when I went to SoC, there was a lot of stuff missing, not to mention that the map was in smaller sections which caused a fair bit of loading. Even with Stalker Complete in SoC, I felt CoP vanilla was a much cleaner and smoother game. Everyone says SoC is better but I just don't see why. --- Drink your milk!