Well, this is a nice question. I must say that I love Mos Espa mainly because you are free and you can do what you want. Also I like a lot Its environment and mission. You can do multikills... always funny. The same in Most Espa Arena.
I like Coruscant too because the suburbs, where there are a lot of poor people asking for food and with sticks waiting for attack you If you dont cooperate, all is old and broken, there are cockroaches in the floors; walls, floors, boxes, etc. are very dirty, I think that the environment is nicely done again. There are like a rodian's commercial district, landing plataforms, offices, a guide tour center and other stuff, a very coruscant environment.
My vote goes to Mos Espa.
PD: I will play it again.