I say 6, due to the fact that all of the cool stuff was there, such as the Altering cave, the egg, and the glamour place, which I forgot the name of. x.x;
Six, only because you could catch Itomaru. And that was the only way for this little fangirl to get herself an Ariados. <3 That island has my eternal devotion for that one simple fact.
One Island. Has the biggest PC and has Mt. Ember which is brilliant to train at. Also has the hot spring/spar thing for relaxation even if old grannies go there. finally, you can catch Moltres an it's pretty cool, if you ask me.