I have been into AR's for almost 20 years (though took about a 6 year hiatus), and I've built a few lowers and changed out a lot of parts. But I'd never actually built one from the ground up until late 2022, and it was this 12.5".
-LMT upper and lower receivers
-Criterion Core barrel
-LMT full auto carrier with enhanced bolt
-Vortex Razor Gen II E 1-6X in a KAC EER MOD1 1.93" mount
-Trijicon RM07 RMR on Arisaka offset mount
Since then I've built the five others behind it, with the middle 16" being a budget minded build I recently put together for my sister in law. The 14.5" was technically a barreled receiver, so I didn't have to set the barrel nut or pin the FSP; and the 11.5" had the FSP pinned from Colt. But outside of that they were complete builds from the ground up.
Hard to say if the 12.5" is my absolute favorite, as I love all of them and they all have their own merits based on the intents and purposes for which they were built. But the 12.5" is my "if I could only take one" setup, so I guess that classifies it as my favorite.