For a SMG, a TSMG, Uzi, maybe MP-5.
Smaller: Glock 18 or Stechkin, Schnellfeuer, Micro Uzi, something like that. Powder Springs Mac 10 .45 and a 21/28 Thompson are staples. Uzi's are good too. M-11/9 SMG is not the quality you're looking for. I have read many a Class 3 collector with the highest grade, sophisticated choppers say the cheap, simple little Sten is one of their favorites and most often used. Suomis are supposed to be very controllable (comparable to MP-5), if you want to start writing your name in bulletholes, and spare parts are extremely inexpensive, even the 71 rd. drums are like 20 bucks.
I like the looks of the Sterling, myself. That is the Rolls Royce of tubers.
Carbine- I'd take a Steyr AUG over anything, selective or semi. No clue how much they go for in auto,but they're out there, however. For a rifle cal/Subgun size weapon, I've seen HK 53's available for class 3 purchase. Although a 14" AUG is just about 2 feet long IIRC.
an American 180 always seemed like a fun toy. How many .22's have you heard of being used by Corrections facilities?
I really think you Need a good ol' Chicago Typewriter. Unless your tastes run to more modern stuff.
Integrally Suppressed pistols/carbines and SBR/S's and AOWs are my taste in NFA stuff, probably will never get anything though.
The S&W 76/MK 760 clones are fairly easy to find and cool as hell too.