I always save inside a house. If I'm on a route, I look for the nearest building and if I can't find one, I'll fly to the nearest town. It's a habit I picked up when I was playing Silver version about a decade ago, and it's still something I obsess about.
I either save in my room, Pokemon centre, but mostly I go into liberty island and victini's room!
They are mostly, some others are pokemon league (When I'm there)
In prof.Juniper's lab by Cedric