You just can't beat a good wooden paddle for a good solid and satisfying thwack!
My favourite paddles have holes in them so that there is no air cushioning, which results in an even mightier thwack.
I have lots of wooden paddles including: thick ones, and one with a rubber strip down it to make it worse! Then I have various thinner wooden paddles too, a long thing paddle and a flat oval ended paddle & more. I also have a selection of leather paddles for those who aren't brave enough for the wooden implements!