Mos Burger sucks ass. From what I've tried the chain of fast food burgers in the honz goes (from best to worst):
MacDonalds -> BK -> Freshness Burger -> Mos Burger
And the best non fast food burgers are Brooklyn Parlor, Journal Standard (both in Shinjuku) and oddly enough Rigoletto in Roppongi, which offers HUGE fucking burgers. Always on the hunt for more though.