The lower serialized part of the rifle that is deemed the “firearm” legally. Given how easy it is for anyone to assemble one, my general advice is just buy a stripped lower and customize it. However there are a few unique options provided by some companies that require you buy the lower complete, or near complete. I intend to talk more about those options here as I get more time. In the meantime you can see refer to the list of AR makers
AXTS: for a truly ambidextrous lower
New Frontier: Polymer lower makes it the lightest cheapest and certainly weakest lower available.
Palmetto State Armory - cheap complete lowers and versatile LPK options.
SAA - cheap complete lowers