MDC and cella are two plainer soaps out there. They both probably have fragrance and a minimal number of ingredients.
I don't know if there are going to be any soaps that are created without sodium hydroxide or potassium hydroxide. I have very sensitive skin, and cella agrees with me just fine, and it's cheap. It does everything that a shave soap needs to do - good cushion, excellent lathering without being greasy or slippery or being a pain to rinse off of a straight razor blade. It doesn't have the high dollar scents that some other soaps do, though.