If it has to be a soap: MWF. Why? Because it's great in almost every way except for being too moisturizing when it's hot and humid out.
If it can be any type of lather: Proraso Green Cream. Why? Because of everything I've used, it's given me the most comfortable and consistent shaves to date. With the Green pre-shave: pure magic.
If you mean one brand? Probably Maria Arman's (Through The Fire Fine Craft/Maggard Soaps/Grim Blades Soaps).
If you mean one soap? Mike's Natural Soaps, either Lime or Coconut. If I absolutely had to pick one, Coconut. I can get my lime fix via Stirling's aftershave balm.
Haslinger’s schafmilch (‘tallow’) is excellent for me especially during the winter months. It lathers great and provides excellent cushion and post-shave feel!