Note that I don't have all of these configurations active at this time. Some of them are actually combinations that I thought were really neat when I swapped an upper and a lower from two different rifles. I'm listing these configurations simply because I think folks who haven't bought an AR might benefit from seeing what configs are working for other folks.
Bear in mind that as a Californian, I must adhere to my state's assault weapon laws, and thus I am drawn to configurations which may make people in 'free states' scratch their heads and wonder "Why doesn't he just use a pistol grip?" The answer is that, in most cases, I'd rather have a detachable magazine than a pistol grips.
M4gery (click to enlarge)
My M4gery feels really light, which has resulted in my wife threatening to steal it from me. There are only two things I'd change about it if I could, but neither can be changed: first, there's a pinned magazine because, well, California. Secondly, the 14.5" barrel has a permanently pinned flash hider. While it's authentic for the M4 platform, I kinda wish it were a muzzle brake instead so that I could remove the pistol grip, attach a fixed stock, and go gripless. I think it'd be a really fun minimum-legal-length gun with a 30rd mag. But since the FH is a permanent fixture, I went overboard with 'evil features' and have every 'evil feature' on it short of a forward pistol grip.
One of the routes we can take to have detachable magazines in California is to not build a centerfire rifle - which leaves rimfire calibers. Constructive possession is always a concern with a detachable-magwell lower, but I feel fairly comfortable because the CZ V-22 upper is insanely hard to remove (requires hammering), and will not accomodate a standard USGI magazine while the V-22 upper is installed.
CZ V-22 22LR upper
NCstar red dot scope. Based on my NCstar scope that broke from an SKS in 7.62x39mm, 22LR is about the only thing I would consider trusting NCstar scopes on.
Short agility rifle (click to enlarge)
This is a mutation rifle formed by swapping around uppers and lowers. But, it was determined that the weighted buttstock and Monsterman grip from my match rifle, in conjunction with my wife's 16" Stag upper and iron sights, resulted in a very agile rifle capable of using 30rd magazines. I was surprised to discover that the rear-heavy rifle nestled very naturally at the shoulder, with a center of gravity shifted rearward in a surprisingly bullpup-like manner. I intend to re-run this test with the the muzzle compensator I installed on her rifle recently to reduce muzzle flip - I expect to see an insane reduction in recoil.
16" Stag upper with my M4gery's LMT rear sights
A2 buttstock with lead weight (shifted balance rearward)
LMT two-stage trigger
True shortest-it-can-get rifle
My wife prefers to shoot the M4gery with its stock collapsed, and greatly prefers short rifles over long ones. However, she also loves her detachable magazines, so being a good husband, I found a way to make her rifle just 31" long.