1) There's not a list of most popular algorithms in the app, but Apple lists that info on their product page for H9 Control: https://itunes.apple.com/us/app/h9-control/id568128041?mt=8
I'm a bit surprised by their ordering though, b/c from what I've seen the top half dozen algorithms by sales have been: Digital Delay, Blackhole, Phaser, HarPeggiator, Quadravox and Rotary.
2) Apple lets users rate apps. I don't recall ever seeing anything though for rating in app purchases.
3) Yes, if we were to come out with a new H9-like machine next year that is compatible with the H9 algorithm scheme, you would be able to transfer your H9 algorithm purchases to the new product.