Clear Sky is the buggiest out of all of them, but yeah SoC does have a few bugs. I've never had any problems with CoP though.
Few of the bugs are generally game breaking or will cause major issues, they're just a little annoying. I think a lot of them are to do with playing on Windows 7&8 so if you're running XP or Vista you should be fine.
CS is buggy as fuck. CoP is kinda buggy. SoC is HILARIOUSLY buggy at times. Like, it has funny bugs, not its funny it has so many bugs.
I've seen a few hilarious bugs in SoC, I've barely played it though, mostly played CoP and never once found a bug in it, at least none that I remember.
Ok I am curious, know of any of the funny bugs in it?
One is the one where you die and randomly get thrown 8000 feet in a random direction. And the knifes secondary attack is a one hit kill on everything. Up to and including a weakened cement wall and tanks if you have mods.