I think the 3rd in his prime was without a doubt stronger than any Hokage before him, but I still have a feeling that the 4th is stronger. The rasengan and sealing Kyubi is pretty impressive, and they wouldn't have so much hype around the guy if he wasn't great. The 3rd even looks up to him, and says that he must sacrifice himself just as the 4th had, and if the 3rd were truly stronger, even though not in his prime, he would've been the one to save Konoha from Kyubi.