What is the fastest single-engine civil turboprop aircraft with cabin seating?
In civil GA aircraft, TBM 900 is the fastest one with a top cruise speed of 330 knots (at 28000 ft).



Source: flightclub.jalopnik.com

However, if one considers all single engined turboprop aircraft, the fastest is the Pilatus PC-21, with a top speed of 370 knots.



"Pilatus pc-21 hb-hzc lands arp" by Adrian Pingstone (Arpingstone) - Own work. Licensed under Public Domain via Commons.