I'm looking for a word that describes the simplest, most complete theory: concise, elegant, complete
an elegant theory
might suit you. There does not seem to be necessary to look for a more complicated word. Some definitions relating to "elegant" :
-(of theories, solutions, computer programs, etc.) gracefully concise and simple; admirably succinct.
-cleverly simple; ingenious: an elegant solution to a problem.
-an elegant mathematical solution--simple and precise and lucid
"Comprehensive" might be what you're looking for. Or perhaps "precise".