Full auto, definitely.
The BAR would probably be one of my first choices too.
Maybe a Thompson.
HK UMP45.
For ultimate practicality, I'd probably have to say an M16 lower. There are any number of calibers and configurations availible just by changing the upper. You could have anything from a full-auto .22LR, a 9mm subgun, the regular .223, to a belt fed upper that looks like a MG42. Also, a "machine gun" is in legal terms a "machine gun", and not a rifle or a pistol, as silly as it sounds. The barrel and overall length restrictions on rifles and stock restrictions on pistols as defined in the rest of the NFA don't apply. You could have a AR15/M16 upper with any short barrel length you wanted, or have it as a full-auto AR pistol, it dosen't have to be re-registered as a SBR or AOW. The only paperwork you have to do is file ammendments with the Treasury for extra calibers it may be converted to.