I've played with the M4 the most out of the lot. It's a nicely balanced alternate tier 2, which is great for expanding commando's options throughout the entire game. It has a neat new feature, firing the second round of a full auto burst instantly after the first, (see respective thread) so that also brings something new to the table. But I've got one gripe; and that's the sound in full auto. The bang noises don't seem to correlate to the individual shots at all, which isn't very intuitive.
I haven't given them all a proper chance yet, though, I can't fairly call a favorite yet. The combat shotgun and husk gun may be major contenders.