The Lake of Rage. I think the name is just plain cool. I like Gyarados as a pokemon and was taken by his dramatic plotline at the mysterious lake. This lake is historic also because it was the introduction for many people to shinies. It also brings you closer to Lance, a charismatic champion that the anime doesn't do enough to showcase.
Another place I like to visit is the Dragon's Den in Blackthorn City. It is a quiet, reflective place, spiritual and private. You get the gift of dratini here, who evolves into yet another of my favorite pokemon. Also in HG/SS you get to know your rival better and form an alliance in this space, as well as feast your eyes on the awesome combined powers of Clair and Lance.
Based on these two places I hope you are catching on that I love dragon imagery :)