PKM and RPK are my favorites, since they have high damage and good accuracy even in long bursts. I've not used the PKM much though (not unlocked it yet).
The MG36 is a little bit more accurate but does less damage.
I personally don't like the SAW and chinese thing. They do little damage and have the worst accuracy. The fire rate is very high though, which some people seem to prefer. As assasin says above, people tend to run when a hail of SAW bullets come at them!