The most overrated speakers in my opinion is the Wilson stuff. I never understood why so many rave about them. To me they sound dry, almost dead, unbalanced and unnatural. I've heard WP7 and 8, Sophia 1 and 2. Sophia 2 is by far the best of them to my ears but I still wouldn't call it good. Heard 'em at shows, in homes, in shops, with very good gear. Most disappointing was the WP8 at a big show, partnered with top of the line Krell Evolution stuff, Nordost Valhalla all over the place, in a somewhat acoustically treated room, playing high resolution recordings by Peter McGrath of Wilson Audio.. That was terrible! Lots of people found that impressive, I don't know.. Perhaps I'm crazy. :)
Of course there are many worse speakers, I'm just tagging along on the "most overrated.." discussion. I'm sure one could dig up some 60s junk that would be unbelievably bad. :)