I would have to say that his best role by far is that of Jake La Motta, although followed closely by a few such as Mendoza (The Mission), Jimmy Conway (Goodfellas), Travis Bickle (Taxi Driver), Johnny Boy (Mean Streets) and esp. Max Cady (Cape Fear). His worst? This boy's life was pretty bad. His acting was good enough but he had a very weird role. (Telling his wife that he'll only have sex with her if she faces away from him is pretty strange). Having said that, I've never seen Rocky & Bullwinkle. :-)
Oh yeah, De Niro's role in A Bronx Tale, (Which I thought was ok actually) is fairly shit, but there's no-one to blame except himself, as he directed the film and so must have deemed it unneccessary for him to have anything other than a small role. Typical father figure role tho, which is what he usually likes to do although rarely excels at. His own dad was a bit crap really and left De Niro quite messed up as a child, so his way of overcoming this was to exorcise his childhood demons by absorbing the roles of father figures. (Sleepers is one of the few times this really works well, although I don't think his role as Father Bobby is quite big enough for the behemoth of De Niro's (method) acting ability.
Jesus I've rattled on here! Sorry if it's bored anyone, I just think De Niro is an awesome actor and I get carried away sometimes. :-)
Ciao!