It's between Sifter and Drone for me. I've never actually played with Sifter but it seems very solid; passes vanilla test easily and results in decent card advantage.
(Isolation Zone is better than either, but 2WW is not appealing.)
We already have a Wasteland to act as a second Wastes, and I don't see ourselves needing more unless we pick up Endbringer, World Breaker or other colorless bombs. Witness the End
I'll go for the Zulaport Cutthroat here. We might even end up tabling the Nullifier if no one else is in UB.
(Isolation Zone is better than either, but 2WW is not appealing.)