Hmm...I would go with Kanto but I'm going with Hoenn since It's so tropical, and has beatiful oceans and there's one city you need to dive to get to it, that's interesting. Hoenn has more nature, great storyline...Kanto has really beautiful places, but the storyline in Kanto isn't that good.
But the best storyline? I would go with Unova. Sinnoh was a bit strange but great storyline and Johto? Really good, you can explore Kanto...yeah.