'Raging Bull' is a cinematic masterpiece. A brutally visceral character study that is aided by stellar performances, beautiful cinematography, an introspective soundtrack, and precise yet abstruse editing.
On the other hand, 'Rocky' is no chump, either. It's a powerful and aesthetically significant work of fiction loosely based on real life underdogs aided by understate performances, simplistic techniques, and a stellar soundtrack.
I'd go with 'Raging Bull' because it appeals to me more on an artistic and technical level, but I think that too many film fans underestimate the cultural significance of 'Rocky', simply because in retrospect it's become something of a cliched story. It's still by far the quintessential underdog story, and Stallone's montages are still as powerful now as they ever were.