How have I not replied to the thread about the OST of my home region yet?!
Here is a list, also specifying whether it is RSE or ORAS:
Battle Brendan/May (ORAS) - it sounds so happy, yet the bass and guitar make it really upbeat and epic at the same time!
Battle/Victory wild Pokemon (ORAS) - ok this is mainly nostalgia. ORAS was my first Pokemon game. I cry whenever these two themes play.
Battle Gym Leader (both) - need I say more than Trumpets?
Route 113 (both) - this theme is one of the most mysterious yet cool tracks in Pokemon history. I remember first going to this route in ORAS and being freaked out by the combination of falling ash, this theme and my first encounter being a Spinda (which was really creepy to me at the time).
Battle Archie/Maxie (ORAS) - wearing headphones makes this theme 1000x better. Especially the intro, where the sounds bounces through both ears for things get extra-intense.
Drought (RSE) - the ORAS remix wasn't bad, but the original screamed creepy with the GBA noises.
Battle Champion (both) - just... damn. The intensity is f**king real. It's upbeat, and suits both Hoenn champions perfectly.
Battle Elite Four (ORAS) - it sounds amazing, especially with the upgraded instruments.
Battle Deoxys (both) - the theme is so upbeat, so mysterious, so COOL!
Battle Zinnia - the ORAS-exclusive track that is so amazing and epic! The violin has always been a favourite instrument of mine, so this epic theme was one I adored!
Azoth - another ORAS-exclusive theme, but this one just made me feel hard. It's so emotional, and makes me think about the manga adaption of RS due to how apocalyptic that chapter was. This one is truely epic.
Per Aspera Ad Astra - did this just become a rock song? OH YEAH!
Strains Of A New Beginning - after a month of playing ORAS, with the adventure finally ending, and the cutscenes that came with it, and that amazing piano... I cried so much. It's beautiful.
Edit: forgot to mention the most nostalgia-inducing track of the lot... for some reason, it's route 110's theme.