I'm more partial to the Mauser k98 myself.

Really, though, if you want to talk 'best', in terms of overall power, it's probably one of the Mk2s. Hell, the prime does the trick imo. You get a mix of all the cheesiest things available:
Easy to use/powerful LRMs.
Gauss rifles, which are better than UACs in most close quarters fighting and have better range/easier to aim
4 ermls for decent beams
and JJs to make all of that poptart
Oh, and good armor.