I know several people have said hands. My favorite implement is a variation on this, fists. I enjoy being used as a punching bag. Being punched in the chest, abdomen and balls with both hands and feet bound, so there is no chance to curl up and hide can be such an intense experience. The bruises the next day can be great reminders as well.

I enjoy the classic implements as well (e.g. canes, whips paddles). But there is something so primal about a sadistic Domme using her hands to deliver the pain she wants to unleash and I yearn to receive. It can make me feel a deep connection.

Knees and feet are a close second.