My own choice, Snatch. I know it's a gangster movie, but there is enough boxing in it, I think, to count it. Brad Pitt nails Mikey, easily one of his best roles, and that final fight scene is better than 99% of others.
Raging bull, Cinderella Man(Russel Crowe), Diggstown (James Woods, Louis Gossett Jnr) and Rocky 1-4 were good. The big man(Liam Neeson), Gladiator(Cuba Gooding Jnr), Undisputed (Wesley Snipes, Ving Rhames) and Million dollar baby were OK.
Not yet a mention of "The Great White Hype," with Samuel L. Jackson, Damon Wayans, Peter Berg, Jeff Goldblum, Jon Lovitz, Corbin Bernsen, Cheech and Jamie Foxx? Wow.