I'm going to offer a hot take:
The lower really doesn't fucking matter much. Today, I can take a spec lower worth half a fuck with a decent fit and plug in all the ambi-shit one could ever need to run the gun well enough. Most are shoving an aftermarket trigger and preferred furniture in these anyways... Sorry but your ambi bolt release isn't that important.
Short of that, you just have to have the appropriate spring/buffer combo (and the buffer tube to allow it).
The "BEST" (*) is:
sub 11" upper - HK 416 (so long as you didn't get one of the post AG/pre BA QC problem children)
11.5 - KAC
12.5 - Hodge Mod 2
14.5/16 - Take your pick...but I've got a soft spot for some old Hodge mod 1's with DD barrels that are fucking tack-drivers.
16+ - That's SPR territory...pick your poison but be wary of the ebb/flow of mfg's.
It's not a 1-stop-shop for most. We're 20+ years from taking Colt receivers/bcgs, slapping in a Noveske, wrenching on a KAC tube and going to town...We just have more options for better or worse.
Those that make good barrels, might have shit rails. The folks making my receiver sets, might not be making my barrels. While, I may not be able to make a gun as slick as a KAC, Hodge, HK, I can certainly cherry-pick a killer gun together if I know what I'm doing and still have a gun that can run cheap(er) parts than proprietary KAC/HK/LMT bullshit...IF logistics are a concern.
Also, saying it again...LMT hasn't changed...just y'all's opinions/exepctations...