As of now, Pokemon Ruby, Sapphire, and Emerald Versions for me. So much epic music in there. You have:
1. your epic Gym Leader, Elite Four, Champion, Villianous Team Boss and Grunt music
2. leisurely Route traveling music
3. a nice, soothing Rival battle theme
4. Frontier Brain music that makes you feel like you're at a royal engagement/very special event
5. Wild Pokemon battle music that somehow doesn't get annoying to listen to and invites a sense of Pokemon creeping up on you and the player's valiant efforts to defeat or capture said Pokemon
6. Trainer Battle music that is epic and still doesn't get tiring.
7. Legendary Music (both Weather Trio and Regi Trio) that makes you feel like you've been submerged in a torrent of sheer epicness. :cer_nod:
Everything in the game feels triumphant. Several themes, noticeably the RSE remixed main theme and the Magma/Aqua Grunt and Boss Battle themes, make you feel at times like you're going on a grand underwater excursion, submerged in the depths and immersed in the water's beauty. Few games can do that with music these days.
Granted, I've replayed RSE many times, so I've gotten accustomed to the music (however, I'm not listening to the Deoxys RSE battle theme until I actually own a Deoxys). Still, I feel that any Pokemon fan, or actually any videogame fan, should listen to and experience RSE's stellar soundtrack at least once.
FR/LG come in a close second. The Gym Leader/Elite 4 and Champion themes nicely reinforce the epicness involved in their respective battles, and a lot of the overworld music is pretty intriguing (Vermillion City, Cinnabar Island, Cerulean City, Celadon City, Seven Island, Trainer Tower); though I don't know if the last two Sevii Island areas I mentioned are using remixed GSC music - I heard that some of the music is remixed GSC tunes.
I haven't listened to music from Generations 2, 4, or 5 because I have not played Platinum, HeartGold, or Black/"Gray" Versions yet (though I intend to). However, I know that the Super Smash Bros. series has medleys for Generation I and II each, as well as a remix of the Dialga/Palkia battle theme. Both medleys and the Dialga/Palkia are some of the most epic pieces of music I have ever heard (you can even repeatedly find me humming the Gen 2 medley every now and then), so I have no doubt that the music in DPPt, HGSS, and BW will be of excellent quality as well. :D