I would recommend taking to Denny at DennysGuns here on the board if you want a stainless match grade barrel.  I picked up two from him and they are very nice.

Quoted: Are these parkerized under the FSB?

That's about as important as the choice in FONT a company chooses to stamp the caliber and twist on the barrel.

I use a pretty basic gothic because that's what MSC had when I ordered the stamp set.
If I could find Comic Sans, I would be all over that...

Quoted: Seriously? I read a while back that moisture could get trapped under the FSB and cause rust. Is this nothing more than an urban myth? What about M4 feedramps. I'm building a 16" midlength that will lead a life of law enfocement so I need maximum durability and ultra reliability.

Parkerizing is a water based solution.
When they parkerize barrels, the barrels are in a boiling hot bath for 5-10 minutes.
If parkerizing solution can't get under the FSB, what are the chances that enough cold water to cause a rust problem will get in there during use?
Is there a little rust under there? Sure, but it's really just a discoloration, not a growth.
Will it ever be an issue? No Way.

Ok, let's say you have found the perfect barrel, all the features you want and 2 different makers sell it for the same price.
The only difference between the 2 is that one is parked under the FSB and one is not.
Take the one parked under the FSB.

Now, let's say that the one parked under the FSB is $20 more, take the non-parked one, it's just not an issue worth even $20 over the life of the gun.
Yes, I suppose if you spent your days in the swamp, that water could get under the FSB and cause rust, but beyond that, it's a non-issue.
The first time you start shooting, you are going to heat up the barrel enough that all that moisture dries out in there from the small amount of gas leakage at the FSB.

Now, M4 ramps GOOD.
You want those for sure.
They are not really required if you have all perfect magazines, but they will save you from marginal magazines should you end up with one.
The majority of feeding issues in the AR are caused by magazines.
Buy good ones and throw them away when they are worn out.

Quoted: As for the M4 ramps, how can I be assured that my barrel will have them? I see a lot of companies advertise M4 barrels where in reality this just means an M4 profile and includes the standard "rifle" ramps not the true M4 ramps.

All M4 barrels are not created equal.
Rock River, DPMS or Bushmaster will certainly sell you an M4 profiled barrel without the M4 feedramps.
LMT and Colt will have M4 ramps, but neither company makes midlength barrels.
By definition, an M4 barrel will also be a carbine-length gas system, not a mid-length profile.
There's no such thing as a Midlength M4 barrel, the M203 cut falls in the middle of the mid-length gas block seat.

Here's MY M4 feedramps picture:



Then there is the BIG M4 FEEDRAMPS THREAD

With M4 feedramps, you also need a receiver with the M4 cuts, so don't forget that.
Your best solution is to buy a barreled receiver, that way you know both parts are ramped and it will already be torqued together for you.

CMMG, Bravo or Denny can likely set you up with what you need.
I don't do chrome lined barrels.
I specialize in custom made match grade target barrels.

Quoted: Basically what I need/want is a threaded 16" midlength with a standard FSB and M4 ramps (does this put this thread back on topic, somewhat?)...

I just caught one more thing you said...

There are 2 different heights of front sight bases, A2 and "F"
The difference being that an A2 is the right height to work with an A2 receiver and the "F" height is correct for a Flattop receiver.

M4 ramped A2 receivers are not very common, the only source being Colt and they are very high priced.
Let's assume you are doing a flattop upper.
You will want the "F" height FSB on your barrel.
Make sure you specify that when you are ordering it.