Particularly the S — the Single Responsibility Principal. So much messy, convoluted code is convoluted because it lacks a singular, clear purpose, and bundles up multiple responsibilities into one section of code, be that a module, class, or whatever is appropriate to your language.

They're all good, and you'll get good insight from them all, but I think that first one is more important and has provided me more value than all the rest. I think Liskov substitutability would be my second pick.