What is the best battle?
Is it against the Elite Four, against Blue or a specfic gym leader?
I think its hands down the all time popular legendary battle with Red in Pokemon Gold and Silver and it got wayyyy harder in HeartGold and Soulsilver.
Epic battle with the 3 fully evolved stater pokemon and the stupid Snorlax that was like a Tank. It was easily the most fun and hardest battle. Also the music was epic. There is saying about the battle from Pokemon fans... "you don't beat red you survive him" lol
http://www.youtube.com/watch?v=1sweo23V9J8