Assuming a replicator couldn't be used to replicate other ST technology, I'm torn between a transporter and warp drive.
Transporters aren't practical for eliminating our day-to-day travel problems, but the idea of placing beacons in space and transporting between them is tempting.
But if we could solve the practical problems of warp drive, I'd take it. Whether we're "ready" for it is a valid question, but you could certainly argue that we weren't ready for it in 2063 either, immediately after a third world war when half of our species was decimated. It was the warp drive that enabled contact with other civilizations, which united our species in ways we couldn't have foreseen.