In Kanto, I like Celadon City, however because of the Magnet Train, Saffron City comes next :P
In Johto, I quite like the feel of Violet City. Also, the Sprout Tower was a good training place for me when I was a wee noob.
In Hoenn, I probably like Slateport City. It is big, and it has a good shopping place.
In Sinnoh, I like Jubilife City. It's big, and I like the television place.