I've got the most experience with FALs so I'm biased in that direction as far as .308 rifles go. They are remarkably reliable. (unless you happen to have one break at the WC3gun while shooting for Team TFL).
Something to keep in mind, I've done a lot of testing with my rifles in sand. (Utah high desert, so not the same as that fine silty crap in the middle east) I've found that any type of semi auto rifle will malfunction when you get a significant amount of sand in the action. The AK works the best, the FAL works pretty good, and the AR works the least well.
However sand in the magazines is the real killer. I found that FAL .308 magazines were very vulnerable, as were AR .223 mags. If you got sand in there you were screwed. Once sand worked its way between the cartridges you would immediatly begin to get failures. I found that 7.62x39 AK mags worked the best in the sand. I'm not sure why, if it is the design of the follower, or the shape of the round, but they could take the most gunk and still function.
In the freezing cold all of the weapons types seemed to work well and I have never had either the FAL, AK, or AR malfunction due to snow.
Freezing rain and hail same thing.
Haven't used an HK much. Have used a Cetme a little bit, but it wasn't mind so I don't think my friend would want me to drag it through the sand, snow, or mud.