Hmm... there's so much about Splatoon that I like, I don't know if I could single out one thing as my overall favorite.
Wait, I think I've got it. It's the world that Nintendo built for the game. The... how do I explain this... not just the backstory provided by the Sunken Scrolls, but stuff like there being a unique language—not just for the Inklings, but for the Octarians as well. Plus that they fleshed out the little details, like the songs being performed by in-universe bands, and the various clothing companies having their sub ability quirks, etc.
I realize this doesn't make much sense... and it is kinda broad for a single favorite thing...