I personally think castration or just literally chopping off their penises is the best way. Because with this method, they won't have the chance to ever do that to others again. Why so harsh? Because rape victims get to live with the damage FOREVER, unlike robbery or others. I believe all people deserve second chances, but rapists should face that consequence of losing their members.

Although I woudln't recommend the same logic to other crimes like for theft, for example. I wouldn't agree that chopping their hands off is the proper way, because that is entirely different as they still have the chance to change and be productive citizens, etc. For rapists, well they can still function well without their penises, it's just that peeing would be different I guess. Moreover, I don't recommend death penalty to for any crime (because our justice system sucks and may just end up killing the wrong people)