I can’t think of a top 10 right now, but I can think of a top 1: Southern California, or California in general. With Vincent Bazillo’s awesome freeware airports in the San Diego area (and a couple elsewhere) and a plethora of good payware in the LA area, I like to fly in both areas. There is just a ton of diversity to the environment from mountains, coastline, islands, farm fields, deserts, cities, pretty much everything is within a short flight which makes it a great playground.
My #2 pick might be Colorado in general. Though it’s a bush paradise, my current go-to for this area is flying the Lear 35A out of the Steamboat Springs area to all of the vacation towns. Tons of interesting approaches and of course breathtaking mountain scenery.