Hardest vote yet for me. Remembrance is probably the best Trek pilot in general. (Not a high bar, I know.) Nepenthe is just fantastic. Not only for Picard and the Rikers, but a lot of the interplay between Rios, Raffi and Agnes is also fun as hell. I'll give my vote to Remembrance, but by a sliver.
As for the Impossible Box introducing technology that should make the Borg unstoppable, I'd like to remind everyone that First Contact exists. You know, the film that gave the Borg time travel, that they inexplicably never use again?
Remembrance - 1
Nepenthe - 1