I Like Slateport city then any other city in R\S\E here i can tell the reasons
Reason 1) there are too many things to see
2) I can buy Protein, Iron and other state upgrade things to raise the base stats of my Pokemon, Its also useful in League
3) There is a museum and a too many Historical Monuments to see like Fossils, models of ship and sumarine and other cool things (Also because first time you enter the museum you get a chance to fight with team rocket grunts )
3) In emerald there is a Battle tent so i can fight with other trainers with other trainers Pokemon it depends on luck that what kind of Pokemon you got.
4)There is also Slateport harbor where i can have a ferry to birth,navel, battle frointer etc etc.
there are more reasons but this are the major reason for why i like this city !