I'm a fan of the Magpul MBUS even on guns that I'm not planning to put optics on. About $39 on the Bay. If polymer scares you and you must have an aluminum rear and you don't care if it doesn't fold, I like the Troy A2 Rear Battle Sight (the original one, not the Gen 2 version).
Good on ya for resisting the urge to put optics on every singe AR you own. There is nothing like the simplicity, compactness, lightness and quickness of the plain old aperture sight.