Splatoon is my favourite game series, I think the ost from both games is great too so here are some of my favourites:
Now or Never, from the first game (the splatfest ver. of this is also good):
Fresh Start, from the second games single player mode:
Fly Octo Fly/Ebb and Flow from the second games DLC:
(I also reccomend watching the Splatoon concerts too)