Spatoon . it's been a while since game music has been stuck in my head like that! Truly unique and different.

Why didn't I see this animation before? It's damn good :)

I'd love now an Undertale anime :P

Life is strange

Undertales and Xenoblade Chronicles X are the best for me. I personally also loved the strong personality and the freshness of Splatoon soundtrack.

Undertale, no doubt.

Is there really any competition?

Obviously not. We just are bunch of morons for thinking that 2015 had lot of promising soundtracks. *sigh* 

Some great suggestions already. Witcher, Bloodborne, Undertale, Splatoon among others. 

I really like this from Heroes of the Storm...

Halo 5 

Tales from the Borderlands - Fantastic use of music to set up the episodes. New song for each episode. They are all amazing. 

Splatoon - Fresh, mischievous and full of energy 

AC Syndicate 

Will think of more. 

I haven't played Undertale, but it does have a great soundtrack. Out of all the games I've played, my answer is Splatoon ( shocking, I know).