My favorite is Burg Rhinefels. What a fantastic, massive fortress and you are free to explore it as you like. Download the flashlight ap for your phone though, or take a flashlight with you to walk through the tunnels. The great hall will amaze you, as will the unique bathrooms. Good view of the Rhine.
The Marksburg is a favorite of many, but you do have to go on one of their tours. The rooms have period furniture, so that is interesting too. The view is great from here.
Burg Reichenstein and Burg Rheinstein both offer tours.
Then there is this lovely little castle, Burg Pfalzgrafenstein built in the middle of the river. Nothing really inside of it, but you can take a boat over to it and go inside.