Best depends upon how much you want to spend. There are 10" screen units by Kenwood, Pioneer and others, that are very very nice and will cost you $2000 including all the harnesses unique to your Prius. If you go that route, it would also be best to upgrade the factory speakers. Go to crutchfield, plug in your car info and you should get a full list of your options.
I would suggest going to Best Buy or a car audio store that has some models on display and powered on for you to play with in person and see how you like the interface / how responsive it is.
