Newhart still takes the cake. Ballsy and unexpected and got everybody right where it counts - even though they were basically blowing off the entire new series!
I hate the Lost finale with a passion 10 years ago, but we are Rewatching it with our (10 year old) son right now so this should be interesting...
STTNG stand out from the pack to be sure.
Also Quantum Leap, even though the ending was rushed and restructured at the last minute. We need more shows like that today.