The Big Goodbye.
This is it... The first of the "holodeck gone insane" shitfest episodes that would mark the modern Trek era.
The episode won all sorts of awards and, in 1987, is actually quite original and fun. It's the strong acting arm of the cast -- Picard, Data and Crusher -- in a 1940's mystery.
What ruins the episode isn't the intervening 21 years since it aired, it's that Trek would overdue the shit out of the holodeck. Arguably, it's the worst thing that happened to the franchise.
Also, is it upsetting that TNG is 21 years old? I think so.