+1 on the Baron BE58. It's fairly fast. Not too difficult to find 5 PAX full load. They can be found on FSE for decent money.
I really LOVE the Carenado BE58 model. It's full featured. Complicated enough to be a challenge without being a total "study" airplane.
http://www.carenado.com/CarSite/Portal/index.phpHowever, if asking about pure FSE v$'s making, it would be hard to beat a Cessna Caravan.
My advice. Find an airplane that you really like to fly. If having to choose between a large or small aircraft, pick the smaller one as FSE operations really favor the smaller aircraft. Once above 10 PAX, finding full loads requires much more flight/route planning.
But most important thing is to find a aircraft type and sim model that you
really like to fly. And then fly that on FSE. Don't chase the v$'s.