If I had to pick one episode to represent the series I would give the edge to City over Tribbles since it much more a feat of Science Fiction writing.
Fiction perhaps, but not science. Time travel stories—especially those invoking paradoxes—are fantasy. "Tribbles" on the other hand is highly "realistic." There have been uncounted instances where Mankind has had to deal with the explosive and unexpected results of tampering with the balance of an ecosystem.
We could probably argue about this to no end but....to me...
Fiction = Fake therefore Science Fiction is either a fake story about science or a story about fake science, I can take either.