The best OST, at least for Pokemon games, has finally reached our ears. There's no argument. A theme as amazing as these are out of the realm of possibility for other games. OR and AS win. 

^This is the best one

That Deoxys theme is amazing. Can't wait to buy this.

I don't know, when it comes to Pokemon battle theme, I'm honestly waiting for something to top these two:

I kinda agree. Giratina's battle theme is the best legenday battle theme.

But the best that came from Gen 4 IMO has got the be this:

Damn, that's a great song as well. Though if I'm being honest, I hardly pay attention to Pokemon games' soundtracks. I'm not sure why though, as I try to pay attention to the OST of every game I play... But there are some tracks I really do enjoy

This one I find particularly kickass.

One of the best tracks from Gold. https://www.youtube.com/watch?v=uv9LH5s4Qw4
One of the bests overall. https://www.youtube.com/watch?v=e_5TVp1wSTU

That's probably the best Rival/Champion battle theme.

I love every single remix, and just found this one today:

I have to agree, these new arrangements are absolutely awesome. Keeping the feeling of the original songs with a few more modern touches, without all the techno stuff that we had on the last games of the series. Can't wait to buy this game,

Wow, I love you. It's pretty good. Not better than the original, considering they toned down my favourite part, but this is still pretty good. So, this is what RBY would sound like with today's technology. 

I will patiently wait for another remake.