I would vote for the Jetstream just for the Garrett TPE331 (can't wait to play around with that engine!), but we're already getting the Commander 690B. Then again, that would mean I'd get to play with two Garretts.
Jetstream or BAE ATP would be awesome. In fact I already posted an ATP request to Justflight. British from top to bottom, an ATP is a legendary aircraft, it's a pity none has decided to bring it to MSFS to date.