Considering only adjustable signal heads...
My favorite 12" heads are Crouse-Hinds Type H ("art deco") and both major forms of Marbelites - the rare welded-backs and the common kind. Round-body Econolites get honorable mention.
My favorite 8" heads are the solid cast GE Novalux and Ornamental Eaglelux. As for modular heads, again it's CH Type D/DT, Marbelites, and GE round bodies for me. Newer Eagleluxes and rodded Eagle flatbacks might be my next choice.