The tiny 6 seater launched Maurer X-Cars are puny and apparently pack a punch.
Kanonen, probably the tightest launched coaster outside of Maurer.
Euro-Star, that crazy traveling Intamin AG invert that got lost in the folds of time.
Basically anything at Grona Lund, especially Twister because of the unique layout that had to fit on top of pre-existing buildings.
Hell, any of the large Zacspins are pretty much the highest track length to footprint ratio of any closed circuit coaster out there.
As for B&Ms, I'm sure there's nothing more compact than Daemonen.