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