Ultimate? OK...
Preban full spec lower (pushpin, magfence, no sear block, milspec pins etc)
Flattop 14.5" 1/7 twist M4 milspec barrel with M4 feed ramps. Either a milspec M16 carrier with Wolf extractor spring or LMT improved bolt and carrier.
Magpul M93 stock with Colt M4 buffer, single point snap ring attachment, strike plate and side rail.
Accuracy Speaks 1 stage trigger or KAC 2 stage trigger.
KAC FF RAS
PRI front sight / ARMS 40 rear sight
Trijicon TA11C in an ARMS 19S
Tritium irons in the above sights (Trijicon/Ashley/etc)
Surefire M900A with blue filter so you dont get reticle wash out - The rail it replaces will be on your side rail on your stock to go back to a normal grip if you want / need it.
PEQ-2 IR pointer/illuminator
PRI Gas Buster charging handle
Replacement pistol grip of choice.
Gemtech or KAC flash hider and the suppressor that mounts to that flash hider. (Gemtech for low cost and legal length on a semi auto, but KAC for a full auto)
You will need a head mounted PVS-14D of course.
USGI Teflon mags loaded with 77 Grain Black Hills NATO pressure ammo.
This gives you a violent fragmentation range of 150 yards and somewhat less fragmentation to 200 yards. Your reticle will match the ammo all the way out to beyond 500 yards which is the furthest you can even make an accurate shot with that barrel. Its 16" in overall barrel length with the Gemtech Bilock so there is no SBR to pay yet its as short as you can get it for CQB work. The TA11 works day or night in all conditions either CQB or long range. You can go with full night vision / suppressed and lower the irons to get the glowing/blooming tritium inserts out of the fieled of view of your PVS14 yet you have day/night iron capability. You can go with or without the front grip/light at the drop of a hat and have both the smallest and most accurate rifle you can get in one package. This is the best Day/Night, CQB/Long Range Precision, Indoor/Outdoor, combo you can put together at the moment. In the future KAC will have a new front sight to consider, the URX2 forearm, a new bolt/carrier/barrel extension and a new flashider/compensator/brake that takes a suppressor to consider. There will be a new magnified optic for the SOPMOD bloc2 upgrade to consider and if you wanted a magnified NV scope there is the PVS-17 or Raptor 4X to consider as a quick swap optic with your day scope for dedicated, outdoor, long range, night time use. Pneumatic buffers will also be developed and need to be tested and considered.
There that is the Ultimate semi auto M4...
Cost? $15-20K depnding on the price you find a PEQ2 and PVS14 at and whether you get the PVS17/Raptor 4X or not.