Part III partisans favor it for what they regard as a more mature storyline in which Marty and Doc both go through some meaningful character growth, viewing Part II as a load of pointless wackiness. Back to the Future Part II partisans favor it for what they regard as a uniquely creative plot that spans three separate time periods, viewing Part III as an unremarkable retread of The Western. Nearly everyone seems to agree that one of them was good and the other didn't quite measure up, but they can't seem to agree which is which.
Others will say it hit its stride with the Powerhouse era, declined, but not by a considerable margin, in the CN City era, fully cemented into its Dork Age with the Fall era and reached its nadir in 2009 with the debut of CN Real. Either it really hit its stride with the Powerhouse era and went to hell in 2004 with the Cartoon Network city era, really got good with the CN City era and started to decline in 2007 with the Fall era, or got bad in 2009 with the debut of CN Real.