I have Colt, Glock and Scorpion mag lowers from 7 different manufacturers.  Some worked 100% from the start and some took some work to get 100%. To me the most important part is the feed ram and angle. Colt and Scorpion have feed rams built into the receiver so helps feeding.  The Glocks do not.  I would suggest a barrel that has a feed ramp built in similar to Taccom or Macon Armory and there are others.  I have a couple of barrels that have circular feed rams and they give low reliability.  On the shelf for now until I get modified.  If you want LRBHO - that is a whole other discussion - LOL

I am a retired tinker so having a gun to make 100% is a fun challenge.

Another vote for Colt/Uzi magazines. My dedicated CMMG Colt style lower has been very reliable for me. I am using a CMMG bolt and Rock River Arms 9mm barrel. I have only used Metalform magazines since they have been 100% reliable for me.

I used a Double Diamond lower and a Spikes upper for mine.  So far it has been basically trouble free.  I am using a H2 buffer and carbine spring but I would like to experiment with  a hydraulic type buffer to try and soften it up a bit.  I just tried my Rex MG7 suppressor on it and it seem to work well.  I don't have a Glock and I really liked the look of a dedicated lower so I went with the Colt pattern mags.

I'm with you there!
All 4 of my Colt-mag builds have been utterly reliable.
I prefer Brownells' Colt pattern magazines. Excellent stuff.

I ordered a new Frontier Armory side charging upper for the C5 lower. I'm building it with a 4" match barrel, an 8" Form 1 suppressor, Midwest Industries 9.25" SP handguard, and A3 tactical "grenade" grip. I also have a Foxtrot Mike 7.5" forward charging upper for my 3rd build. The side charging feature on a short weapon feels more fluid and natural to operate than a standard rear charging handle (IMO).