I can't decide on one. So one per region.
Kanto: Celadon City. I love the big city. It's full of greenery, lots of busy stuff going on and it's at the centre of the region.
Johto: Azalea Town. Love the heavily forested theme. Smell of burning wood logs, and an awesome comfy forest to explore.
Hoenn: Lavaridge Town. Cozy small town. In the mountain around the middle of the Hoenn land mass. Hot springs is so relaxing. I like the dryness of the surrounding area, I like dry heat!
Sinnoh: Solaceon Town. At a crossroads with convenient access to many areas. Love the Sinnoh wilderness surrounding the Town. Comfy Route 209 to the south, with it's many streams. My favourite Sinnoh route 210 to the north, with foggy mountain canyons to explore. There's a cafe nearby and a daycare. Not to mention cool ruins to explore nearby.
Unova: Icirrus/ White Opelucid/ Lacunosa. Hard to chose. I like Icirrus becomes it's the one pokemon city that most resembles my irl home, surrounded by marshland with warm summers and cold winters. I love the atmosphere of White Opelucid, like an old european town it reminds me of those old comfy pokemon movies when Ash and co. explore beautiful old cities. So cozy. Lacunosa town for the same reasons as Opelucid
Kalos: Laverre City. I'm also a fan of Camphrier and Dendemille but Laverre takes the cake for it's mystical atmosphere, remsembling my home and beautiful scenery surrounding the town. I love the outter aesthetics of the factory too.
Alola: Seafolk Village. I didn't mention before but a lot of close seconds to many of the regions are ports, if they are old cities and beautiful. Seafolk capitvated me because it was a village whose beauty surprised. A comfy front door to explore the wilds of Poni island. And you could take your home boat and sail anywhere.
