Off the top of my head, I can think of four outright violations of the Prime Directive in TOS. One may surprise folks:
1. John Gill in "Patterns of Force".
2. Ronald Tracey in "The Omega Glory".
3. R.M. Merik in "Bread and Circuses".
4. Jim Kirk in "Miri".
Does Merik count, though? He's not Starfleet, and thus (one would assume) not bound by Starfleet directives.