Well since I can't try mantine surfing yet... I'd have to say for now the improvements to the rotom dex, I like it a lot more now xD
The acknowledgement that generations between the first and the seventh exist. My first encounter being a Buneary was very gratifying.
Mantine surfing is loads of fun! It's my favorite new feature I've tried so far. I think the new photo options are cute too.
Probably Mantine Surfing, is a fun and easy way to earn BP and not as time consuming and tedious as the Battle Tree. Also love that you can play with different kinds of pokémon and have them follow you around is a really neat addition even if it doesn't do much for gameplay.