I'll stick my neck out and say that the best overall car wash soap is Griot's. It's the standard against which I compare all the other soaps I've purchased in the past and continue to purchase (in hopes of finding one even better). I haven't found one yet that's as well-balanced in terms of slickness/lubricity, cleaning ability, and sudsing, while still being mimimally harsh on lsp's. It smells great, too. To me, it's the soap everyone should try at least one time (as plenty people have) before deciding what they consider to be the best.
(And really, if it's good enough for Accumulator's wash regimen...)