Lot's of S3 eps. being mentioned and it's not just a coincidence.
I initially though "The Defector" as well. "The Enemy" is a good choice too. Hell, any of the S2-S4 Romulan episodes would be a good mix of action and drama (i.e., "The Mind's Eye", "Contagion").
"Tin Man" could be good too. The essence of Trek right there.