I would say either the m16 or m60, both with rapid fire. The m16's reload has always been so slick and rapid since cod4, and the m60's is just badapples with all the hand movements you perform and  banging the side of it at the end.

I agree the G36C has a pretty nice looking reload, but i like the CM901 reload, especially when your mag runs out and he lifts up the gun, hits the mag in, and clicks the button!

The model

i'd say the FAL (mw2).  i liked how one just knocked the other out.  for mw3 i'd say the G36C or the SCAR-L.