As a child, my dad's paddle and wide belt both hurt like dickens. Comparing them is a close tie. I think the paddle might have been a bit worse: there was something about the sting of the wooden paddle that really gets to you even worse than the belt. But as a kid I never could really compare side by side, since I got the paddle and belt at different ages, however such were my memories of how it felt, and later roleplay re-enactions seem to confirm it.
But the worst of all were bare-bottom switchings from my mother. Luckily for me, spankings in our house were almost always given by Dad, and he did not use a switch, only Mom did, and Mom spanked only on rare occasions when Dad was away on a work trip and some particularly egregious misbehavior required immediate discipline that could not wait for a few days until Dad comes back. I got only a few switchings throughout the childhood, but they were very memorable and in a league by themselves.
Nothing ever compares to the anguish that a slender fresh-cut whippy switch inflicts on a naughty boy's bare bottom, and nothing penetrates a naughty boy's mind like a sharp searing sting of a switch across his bottom! I remember clearly when I or my brother were spanked with a switch by Mom, even the pitch of our yelping was very distinct and different from when we were spanked by Dad, much higher and with even more intense tone of urgency. But for all the sharp spanking pain my mom's switchings caused, they left no "damage" except, sometimes, fleeting puffy pink lines across our bottoms, which were gone soon, but no soreness of the bottom like my dad's paddlings produced.
Also, luckily, I did not have to cut a switch for myself, my mother always did, so at least I was saved from the mental agony of having to cut a switch for myself, although not from the agony of a bare-bottom switching.