The traditional Schwinn cantilever, aka the "1955 Cantilever" is a design that, IMO, looks nice, and it also has the benefit of having a lotta room just about any engine, even for the big block monsters like the Whizzer 138, Briggs, and HF212. They have so much room, that a chinagirl looks puny in em and often takes a custom front mount because of all the space. My first build used the '55 cantilever, but in aluminum, for example:
Edit - the only problem with Schwinn cantilevers is the rear dropouts tho, they are almost always front-exit/edit
I agree that a 79/97/99 is perhaps the best class of engine tho, if you spend good money on a clutch. Those engines produce nice lazy torque - double the torque of the high-rev engines at a low RPM - and they aren't over 28lbs like a 212, not to mention the bike needed to support such a beast!