My favorite would likely have to be the Pokémon Center from Omega Ruby and Alpha Sapphire.
This one seems to be very big, generally, which does seem to be quite fitting for a place like this. Logically, locations such as this are meant to be kind of huge, as people are always coming in and out, and there would likely have to be enough space for everyone to breathe. It's relate-able, so to speak.
Aside from above, I'm also pretty fond of the Pokémon Center from Ruby, Sapphire, and Emerald. Although this one's interior isn't as huge as above (and likely doesn't make much sense to some for it being small and whatnot, due to reasons I've already stated), I still enjoyed its simplicity; in terms of this, it wasn't so bad.