My personal favorites are the rarely-seen displacement badges used by American Motors on their V8s of the 1960s. The AMC 327 and 390 engines often get mistaken for Chevrolet and Ford V8s of the same nominal displacements, respectively, but the hard-to-find Rambler 287-cubic-inch V8 badge makes my day when I find one.
The metric system is all scientific and stuff, but would Shakespeare have used it? Hell no! Still, once you get above 8 liters, as in the case with the 1970-76 Cadillac 500-cubic-inch V8, displacement-badge coolness is attained. What's your favorite?