In a tent in front of the Sea Cottage.
Seriously, definitely Goldenrod City for the following reasons:
1. Yeah, gotta love that music.
2. Easy access to both the train station and the day care center, as Oni pointed out. Mostly the train station, if I ever get bored of Johto (which is heavily unlikely).
3. Department store. Yeah, I know it makes me sound like an incredible girl, but if I ever need to buy anything, chances are, Goldenrod City has it, either there or in one of the other businesses in such a massive city. And if it doesn't, then I probably don't need it.
4. Grooming salon for my Pokemon.
5. If I ever want to spend a little money and have a bit of fun, I could always go for card flipping.
6. Radio Tower, complete with a radio program that gives out items for easy-to-win contest prizes. (Yay stocking up on random stuff!)
7. Trainers and a labyrinth underground. How can I get bored in a city that's got a lot of places to explore, above or below the pavement?
8. Easy access to the National Park for pretty views and bug catching contests.
9. Bill's family = chance to meet Bill. After all, he comes back to Goldenrod City every once in awhile, right? (And I'd rather not be molested by people on Nugget Bridge, kthnx.)
10. Pokemon gym right there for both training and making fun of the girls.
11. "Festive city of opulent charm." Wonder what it's like on Halloween.