Re: What company makes the best M4 Style carbine??
Get a Colt, then you never have to wonder.
They've made more, and for longer, than all the others combined.
They have a standard they meet, and test, at every stage of manufacture, both parts and assembly.
Everyone else, that is any good, has gone to the COLT school (or military, written by Colt) to learn how to build them.
Ever hear of a Noveske school, or LMT school?
Most of the others are assembled by people with little, or no, proper training or test equipment.
Can you build one yourself? Sure, and maybe better than most of the commercial rifles available. But you (or most manufacturers, ie, really assemblers) will not have anyway to properly assemble and test each stage of the work.
All my "go to" AR's are Colt.