I don't really re-watch anime but back when I had no internet and only 20 first episodes of Shippuuden I watched Deidara vs Gaara a thousand times out of boredom :|
Also since the only parts that I do actually re-watch are those with Fu and Torune, it would make their "fight" with Obito and that filler flashback sparring match my most re-watched fights :|
In the manga my most re-read part would be entire Part I (with all its battles naturally). My favorite fight in the manga would still be Naruto vs Pain probably.