I would say it was the humiliation that was the worst. I attended only schools where the Scottish tawse was used (we called it the strap, in most state schools it was called the belt).
We were always punished in front of the class, and I am positive I must have gone a very bright red every time I got the strap. (In fact, friends would tease me about that, which only made me even more conscious the next time.) I was embarrassed about the whole punishment, acutely aware of everyone staring at me and watching my reaction, worried about how many strokes I would get (we rarely knew in advance), worried about keeping my composure and being able to put my hands up for the next stroke.