Here is a list of the parts that I like for a  M4 build. These are the most bulletproof parts I can find. I like to hear what everybody likes and maybe we can get a concensus. At least on some certain parts that are the best.

Colt 14.5 Socom barrel
Colt flattop upper, Bravo Company gets a close second.
Muzzle brake, No favorite. Yankee hill, Viltor, Vortex, all good as long as it makes the barrel leagal length
Gas tube, Colt or Bravo
BCG. Bravo's full auto. No exceptions
Charging handle, Gas Buster with the "Big Weenie"
Rail system, Daniels Defense Omega
Vertical grip, Tango Down with the quick release latch. Expensive but easy on and off and secure.
Lower, No favorite but Preban. A collapisble stock makes the gun alot better for fit.
Lower parts kit, M&A get the Single stage trigger
Rear grip, Magpul Miad. standard kit.
Trigger guard, Magpul aluminum enhanced.
Buffer tube, Milspec size
Buffer, H model
Buffer spring, Carbine size
Buffer tube nut,
Buffer lock plate, single point Ambi
Rear sight A.R.M.S. 40 standard
Optics, Aimpoint Comp ML2. The lowest price one works just as good as their higher priced ones. Just alittle less battery life.
Optics mount Larue 150
Stock, Magpul CTR
Light, Surefire 6p led
Light mount Larue 606
Magazine, Magpul 30 round PMAG with window

On the part with no company names. I would probably go with Bravo company or PK firearms

Looks good but if I were building a rifle from the ground up (which I am), I would use the DD Lite rail instead and flip up front sights. JMHO

The only nit-picky thing I would change is the rear BUIS. I run both the ARMS #40 and Troy folders. While both are excellent the Troy is a better product. It's a simpler design with fewer small parts.

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ETA: Here's a pic of each BUIS on two of my carbines:

Why a ARMS BUIS instead of Troy?

C_Jan beat me to it.

Also check out the SDI extended A2 FH and VLTOR light mount.

+1 on the Troy buis
Also maybe a Larue Fug instead of the tango down
Might want to consider the Larue cantilever mount for the aimpoint just in case you ever decide to add a magnifier to it.
Everything else looks good

Obviously this isn't a budget build, so I'm going to say throw in a VLTOR MUR instead, and a MEGA Billet Lower.

Now, if this is something you're actually going to build and not a dream gun, then what you have listed there is definitely GTG. Theres not necessarily any need for billet recievers, but they'd sure be nice if you're looking for the most rugged build possible.

I have a similar build.
Not sure why you'd want a QD VFG though. You plan on switching it between weapons often?

I really like the lever attachment on the Tango down grip. It's easy to take on or off and Locks like a rock. If you ever used Tangos other grip you know what a two hand operation it is. I didn't think of the Larue. That's similiar and you can change the length of it. I don't think it comes in colors.
I thought I read some place that the Troy sight has play in it when it's locked in place. Is this true?

I own 3 Troy rear backup sights and 1 Troy front backup sight. None of them have any play when deployed

Current Troy on left, older version on right:


Front Troy sight (HK style):

If you're going for "bulletproof" why the M&A LPK?  I'd go for a Stag from Denny, Anvil, or if I'm really feeling like going for expensive and bombproof a Colt from SAW.  OR just buy a comple BCM lower when they're available and add a BCM M4 upper.  Done.

Looks solid, but if I had to come up with a critique:

I've rolled the switch on many a ML2  to find the batteries had shit the bed.  I'd say the extra coin is worth it for the newer Aimpoints.
Polymer triggerguard does the same job for less cash and weight.
There are better backups out there than the ARMS 40.
Why the lock plate instead of the Noveske QD or the ASAP?
Single light in a 606?  2 is one.
SOCOM barrel is heavy.
TD VFG over the FUG?
H buffer not necessary.
Better LPKs out there.
No fave for muzzle devices?  They all perform differently.  Try some out .

Post some pics of whatever you end up with.

Nothing wrong with the ARMS BUIS. I've heard of a lot more issues with the Troy than the ARMS.

Swan might be a Dick, but they are fantastic sights.

The hate for anything ARMS is strong here.

BTW I own both, you'll be pleased with either. Also had/have  PRI, GG&G , MI, Matech. all are good BUIS.

Edit- MIs SPLP is the only one I'll buy for future use. Not going to just dump everything else, but the SPLPs are really nice, and cheaper.

The MUR is an upper receiver, not lower from what I can tell.

I'm pretty sure he knows that, "VLTOR MUR (upper) instead (of the Colt or BCM), and a MEGA Billet Lower (instead of a forged unit)"

Sounds like a nice build but since you're looking for critique:
- I too would go with the Troy > ARMS
- Vortex FH or simply SDI extended A2
- FUG > TD QD
- RRA LPK > M&A I've used both and had to slightly modify the M&A, the RRA went in flawlessly
- I prefer an H2 buffer. I put one in my LMT 14.5 lightweight/KISS and even my girlfriend, who is very picky about recoil, likes that rifle.
- I'd go simple on the lightmount, Vltor offset
or
You could just wait for a BCM 14.5 upper and get a BCM lower from Denny's to simplify things. I'm pretty sure most would agree that would qualify as a "bullet proof" setup!

Here's my contribution, just my suggestions on my preferred parts, there are a lot of bulletproof parts available and the final decision would be your own personal preference.

Rail system: DD Lite Rail/ RIS II/ LaRue
BCG: Colt M16/ LWRC/ S&W
Bolt extractor upgrade kit: BCM
VFG: LaRue FUG
LPK: Colt
Trigger: Colt 4# duty/ field FCG from Bill Springfield or Geissele SSA
Rear BUIS: LMT/ LaRue/ Troy
Sling endplate mount: Magpul ASAP ambidextrous sling attachment point/ Noveske QD endplate
Optics: Aimpoint M4s with LaRue QD mount
Stock: Vltor EMOD
Light: Surefire M600 series Scout Light/ M96 series with LaRue QD mount

Everything on your list looks fine except the only one that I would strongly change would be your rail system.

The most problems of rail systems is that they get loose except of the type with an additional locking design which prevents the rail from getting loose, the ones I suggest have the extra locking design.