For me it's Twilight with Azati Prime as a close second.
I do love the character interaction in Twilight. Archer's tough, overgrown boy scout veneer is broken and we see a vulnerability that is underneath. And T'Pol's loyalty is beautiful.
But it is the visual impact of these two episodes that leaves a lasting impression in my mind. The opening scene of Twilight and the closing scene of Azati Prime were both stunning. Perhaps the most memorable (visually) moments of the entire series.