Hoenn was my first region and I honestly love all of it, although I did hate the cave music when I was younger. (Shout out to seven year old me managing to get stuck on Route 110 because I couldn't see the grassy path below cycling road)
Routes 119 and 120 have the best soundtrack and are so pretty, not to mention fun with all the different paths/bike areas. I also really liked Mt Chimney and the Jagged Path, they just felt exciting! I also loved hopping down the mud flats in Meteor Falls
I have a visceral nostalgic reaction to the Oldale and Verdanturf town themes in particular, but as a location, you can't beat Fortree. I think my favourite gym is actually Wallace's from RS, the ice floor puzzle was really neat. I still remember the first time I solved Winona's swinging gates.
I think part of why Route 120 is my absolute favourite place is the reflective puddles. That's the area where they feel the most fun, with the rain bouncing off all of them and splashes as you run through.