For a water turtle kept indoors, one of the best are the mud or musk turtles. They stay pretty small so you don't need an enormous aquarium that takes up half your room, they don't do a lot of basking so you don't need an expensive light setup and they're quite hardy as adults. The babies can be kind of fragile though so it's better to start with older turtles.

I keep a few stink pot turtles (Sternotherus odoratus) which I really like.