Which is the best site to watch summer fireworks in central Japan?

Definitely try the Sumida Fireworks, over the Sumida River in Asakusa, Tokyo. It's super crowded, so you should try to get a hotel or Air BNB with a good view to watch it from there. Or watch it from a restraunt -- perhaps the Asahi Resturant or even SkyTree would let you have a good view! 

I highly recommend the Arakawa fireworks because I find it the least crowded. There are a lot of fabulous viewing spots along the stretch of the Arakawa. People set their own picnic mats and eat their bento while watching the fireworks. We often view it from the stretch of river near Kosuge Station. Enjoy!

For me it's Atami fireworks. Very nice view and the sea is close.