Well, I've ridden a lot in California, and in NY, MI and Ohio - and 20 other states. NY is great, but not the variety of California. California is great, but I did not cycle anywhere that had the feeling of CNY finger lakes or the Adirondacks, and nowhere else I know of will you find a bike route as long as the Erie Canal. Michigan beaches are cozier than California's and there a many more organized tours there than most states I've seen. Ohio is my home state - I still love riding the east central hills and There's nothing quite like TOSRV. New England can't be beat in the fall if you avoid the traffic, and I loved being able to follow the river through Vermont on a hot July and dip into the water every once in a while.
In short there is no best but this thread could be useful for those wishing to explore new territory on a tour.