For my money, "Frame Of Mind" is the king of them. An excellent performance from Jonathan Frakes is icing on the cake for a story where we, along with Riker, begin to honestly believe that Number One has gone insane. Still gives me chills every time I rewatch it.
Another vote for "Frame of Mind" here. Gives me such chills that I don't watch it any more.
I don't watch "Violations" for the same reason, but I don't think it's that good an episode anyway.