Mine is the one from the second generation, because that's the game I got first (gold version). However, my favorite battle theme overall is either Cyrus theme from D/P or Team Plasma's battle theme from B/W.
Ohhh... I like the rival battle from HG/SS and the original trainer battle theme is classic and great, but my overall favorite would have to be Cynthia's battle theme from D/P/P!
Hmm.. Probably the Kanto trainer battle theme, but from GSC. So refreshing after spending so much of the game in Johto and getting used to the music there.
Red/Blue/Yellow all the way. Everything about Generation I is pure videogame bliss. The soundtrack's epic scope spans form your very first rival encounter to the final showdown with the champion.