There's a whole forum dedicated to "Ultimate" 10/22s over at www.rimfirecentral.com.  What does your "Ultimate" AR15/M16/M4 build look like.  Here's what I'm thinking at the moment:

* 18" or 20" Kreiger .75" profile stainless barrel w/Wylde chamber (accuracy and reliability)
* M4 barrel extension (reliability)
* Smith Enterprises Vortex G6
* Colt bolt group (strength and reliability)
* Stag/CMT or Mega flattop upper and M4 lower receivers (value for the money)
* Stag/CMT parts kits (value for the money)
* Accuracy Speaks single state trigger (accuracy)
* Forged receiver extension (strength and reliability)
* DPMS carbon fiber free float tube (value for the money)
* Stock and pistol grip (TBD)
* Harris bipod
* TA31F

I already have an A1 knockoff and I'm working on another.  I also have an 5 lb. 4 oz. carbine.  This would be more of a precision rifle with a minor in tactical.

If I substituted a Douglas barrel, OPS Inc. muzzle brake and PRI float tube, and added an LMT SOPMOD stock and the right backup iron sights, I guess it would be a fair SPR approximation.

I like mine right now;

Sabre Defence 1/7 Mid-length Gov't profile barrel (bullet weight options)
CMT M4-cut flattop upper (to mate up with the M4 barrel extension)
EOTech 512 (I have no NV, nor will I in the next 5 years)
ARMS 40 (I like it)
Regular hand guards (no frills and functional)
RRA lower with standard trigger (same)
6 pos. RRA OD green stock (CMT mil-spec 6 pos. M4 stock would be on there, but they don't have an OD one)

Maybe if I went crazy, I'd throw a DD FF rail on there or even an MI 2 piece 4-rail for a VFG and a light mount.

Obviously, bells and whistles aren't my forte

WIZZO

ETA reasons in parenthasis.

I've actually been doing quite a bit of homework on this one...

Bushmaster Carbon-15 lower (weight)
Foxbat Grip (Comfort)
White Oak Armament Geissele Trigger (reliability and flexibility)
M4 SOCOM Telestock (flexibility and weight)
American Spirit Arms Side Charging Upper Recever (conveniance)
Hard Chromed Bolt and Carrier (reliability, durability)
Obermeyer 16" heavy stainless barrel, fluted with a 30 degree target crown 1:8 twist (accuracy)
DPMS Carbon Handguard (heat control and weight)
Dual Picatiny Rail gass block (iron sight and flashlight mount)
Surefire M3-CB Flashlight
American Spirit Arms Flipup Iron Sights (Backup)
Leatherwood Hi-Lux 1x50 Red Dot (tactical, plinking, or real world)
or switch with
Leatherwood Hi-Lux M-1200 6-24x50 (target shooting at range)

This is my actual project that I'm starting over the next few months. This is a light weight tactical carbine that's capable of 300+ yard shooting (planning on loading 77 grain HPBT's). I'm probably going to paint it urban digital camo, just for the hell of it.