On the assumption that "rare" means aircraft that are actually still in service here in the U.S., I'd say the 737-300 and the DC-9-50 would top my list. Southwest still flies the 737-300 and Delta still operates a few of the -50s. Although I wouldn't call it "rare" just yet, soon enough, the MD-80 will disappear from American skies - probably in the next 5 years or so. Even the early A320s are starting to be retired by the majors, though they may still soldier on a few years longer with the second tier carriers.