I want to put a new bolt carrier group in my Pre-Ban HBAR, who makes the best and can I just "drop it in"?

LMT, yes

SORRY, WHO IS LMT.

Lewis Machine and Tool.  I suggest a search on "LMT" and "Lewis Machine"

Les Baer or Young Mfg.  Actually, the L.B. carrier is made by Young under the same patent number.  

Next best is Smith Enterprises.

You can just "drop them in", however it's always a good idea to check headspace to assure that your bolt/extension/chamber combination does not result in incorrect headspace.

I have all those sitting in front of me including the LMT.  I will give you my opinions.

1. Les Baer AR15: Bolt Carrier is hard chromed and of non stadard dimensions.  The relief cuts running up and down hang over the firing pin retaining pin and it prevents removal of that pin without an improvised tool.  I give it a 10 in finish.  Its amazing.  The bolt and other small parts are parked.  Great machining there but not as good as the carrier.  I give it an 8 or 9.  The fit is tight... almost too tight.  Although its A15 carrier it weighs possibly MORE than my Bushmaster M16 carrier.

2. Smith Enterprises M16:  Holy crap hard chrome never looked so bad!  Tool marks everywhere.  Cam pin difficult to remove.  The extractor is textured?  Why?  Bolt machining is good.  Not tight but not smooth.  Why would anyone buy this?

3.  Bushmaster M16:  Way better than the Smith but not as good as the others.  Exactly what you expect from a high quality Milspec part and Milspec finish.  Was tight and is now very smooth broken in.  Loosest fit of the bunch but utterly reliable.

4.  LMT:  New Design.  Designed for enhanced reliability.  10 finish on carrier and 10 finish on bolt.  WOW! The top of the rear shelf is angled on the carrier.  Why?  I dunno.  3 Gas ports (smaller) to vent pressure in carbine gas systems.  Carrier is Milspec finish the bolt is electroless nickle.  Double extractor spring system, radically modified bolt lugs. Bolt slides almost as easily as my Bushmaster but has less play.

Although the Les Baer unit has a prettier finish and the nice LBC engravings, its no better than the LMT although the LBC is possibly heavier even though its an AR15 carrier and the LMT is M16.

How does the Colt M16 group match up against these?

SinistralRifleman had some words of wisdom that went soemthing like this...

"Trying to compare who is the better mfg'r is like arguing over which one of your cousins is more attractive.  The AR industry is so inbred and but a handful of mfg'rs makes everybody's parts."  

Not an exact quote, but you get the idea.

Other than the LMT, they're all the same.  The problem is no one replying here has 25 of each brand to test.  Every barrel has the bad appple.  Its something you will never escape.

I respectfully disagree. Having fractured a handful of hard chromed bolts and seeing extractor spring weights all over the map I would submit that not all parts are the same.

A Colt or Bushmaster bolt for example that has been shot peened and polished during assembly is going to hold up better over the long term than one that wasn't.

Of course there are defects and occasional failures with everything, but some are more prone to failure than others.

EDIT: I'm aware that outside vendors supply many of these parts, but my point is that not all vendors are created equal.

LMT is nice, but expensive. The new RRA "enhanced" unit looks interesting.
I had a Smith Ent., but the carrier cracked inside the firing pin ramp, so I replaced all my carriers with modified M-16 carriers.

The Young/Les Baer carrier is a specially designed unit different from all others.  LMT has a different bolt AND carrier.  These are not similar to each other.  My Colt M16 bolt and Carrier were just like my Bushmaster except they had the C and MPC marks.