For full disclosure, Yesterdays Enterprise is in my all time top 5. I love this ep and highly recommend it. But within the parameters set down by the OP, it doesn't fit the request. I'm not trying to pick nits here, I'm just pointing out the fact that it was actually a very Klingon centric episode even though actual Klingons were never scene, just their ships. In the timeling created by the E-C's arrival, the Fed was about to lose a war to the Klingons that should have never started in the first place. Capt Garret was killed by a Klingon attack. I could go on, but you get the idea. The entire plot of Yesterdays Enterprise is woven around the Klingons. Another great ep. The problem is the Klingons play a very important role in Part II and they, or their ship, have plenty of screen time. I guess my vote would have to go to Sarek. It has a little of everything. Good TOS cross over. All of the main cast gets some time, and Stewart still gets to show off his acting chops. The Perfect Mate would be a strong alternative. Normally I would put The Inner Light in contention as well, but if you are chosing just one TNG episode to watch, it might be a little too Picard centric.