Sam,
Currently Colt is producing the M4 which is replacing the M16A2 in many Army units. It is a collapsible stock, removable carry handle, 14.5 1/7 twist.
Both Colt and FN produced the M16A2 for the Army. Bushmaster produced a small batch of M4s for the Army several years back.
If you want one as close to an M16A2 as possible, you'll have to get a pre-ban Bushmaster. Colt has been using bigger hammer and trigger pins and has installed a sear block to prevent the installation of M16 fire control parts. Colt also uses a two screw arrangement for the front push pin, some of the Colt lowers are drilled so that you can replace the two screw arrangement with a push pin and some aren't.
I'd like to state that the changes Colt has mad in no way effect the function of the weapon. It was a marketing decision to keep the rifle off of various ban lists.
If you want the absolute closest to an M16 go with the Bushmaster, however most people would never know what the changes Colt made were, unless you pointed them out to them.
As for who makes the best AR15, that's a very loaded question. IMHO Colt produces the best. I have 6 in various configurations, 3 Colts, 2 Bushmasters and an Essential Arms. I am satified with all three different makes.
As for barrel length, that depends on how you intend to use the weapon. 16" carbines are very handy for CQB and police patrol use, but you lose enough velocity to degrade the performance of both M193(55gr) and M855 (62 gr) out past 200 meters. Not critical if shooting varmints but might effect your choice if you foresee using the weapon in a combat situation. (most small arms engagements are 300 meters or less anyway so that's not as much an issue as it seems)
HTH
Jeff
You know, if I had a 20" upper and fixed stock, I could change the shorty over in about 15 minutes and have a varmint rifle. The you find out that if you just picked up a lower and parts kit you'd have another AR and not have to worry about changing it over.