All the islands have their conveniences and inconveniences. Guess you need to make a choice on whether you want more "city" or "rural". I live here on the Big Island. We have Ironman route, we have two 13,000 foot+ mountains with lots of other Off-dirt choices. Then we have some pretty good mountain biking stuff.
I know Oahu's got a lot of choices but lots more crowds. Maui's got a lot to choose from.
Good luck with your search. If you make it here to the Big Island, give us a jingle.
N. Shore (Oahu) has a lot of mountain bike trails (and plenty of other stuff to do). I don't really ride road bikes but there are certainly plenty of scenic rides to do on pretty much any of the islands.