The Dreamcast classic Rez is the only answer. No game has ever fused music and gameplay so perfectly.

Fear is the mind killer...

I personally loved the first game (I really, really, really loved it :p), but tbh I don't think it's for everyone. Looking at your avy and sig, you might like it though.

Demon's Souls, Assassin's Creed II. AC II was so good in it's music the devs have used the same music (almost) for the next game, and probably for revelations too.

Yakuza 3 had some epic moments too.

Also, Bit.Trip Beat. I loved the way the music was done in that game. It was an incentive to play well, and it was so well arranged and coordinated. It made it a joy to play.

Flower's music gave the game a nice ambiance, but it was a bit too subtle for my taste.

I enjoyed the upbeat and lively soundtrack from de Blob more. I especially loved how the music would get more intense and more instrument layers would be added the further you got in a level. I feel like playing it now...

Ocrina of time???

I'm a big fan of sort of 3D platformer/collector/action games.  Watching a couple of reviews, I think this game could be bang on.  Thanks :)

I came to post this. Thanks for saving me the time and effort.

Wait a tick. That took longer to type than if I had just said, "Rez." Thanks for nothing, you jerk!

Superbrothers: Sword and Sworcery was a pretty funky audiovisual jamathon. Jim Guthrie is a pretty chill groovemaster and stuff.

Also, Okami had pretty great music if I recall correctly ^.^

I would agree on both Demon's Souls and Assassin's Creed II.

And Oblivion and Skyrim of course. They don't just have special battle music, but song lists that play only in cities, or during exploration or in dungeons.