I love Route 45 and 46. It completes the loop of Johto really nicely and it uses the Route 34 music as a nice throwback and to signify that you're going back home, to finish your journey. I love how Dark Cave connects to Route 31 too. Lots of trainers to grind up for Clair, and you can catch Skarmory, Gligar, Donphan, and Ursaring here.
But my favourite part is something I came to realise only a few days ago and it blew my mind: The 1% Phanpys in Crystal that appear on the bottom of Route 46 (the part that connects with Route 29) are ones that rolled down the mountain and ended up at the bottom. And the 1% Teddiursas in Dark Cave are ones that wondered into the entrance on Route 45 and got lost on the opposite end at Route 31.
Overall just a kino Route. Looks amazing in HGSS too with the river going down the middle.