Well, I was thinking the DA-42 until you mentioned maitenence costs and reputation. We have six and can only keep one or two flying at any one time. Based purely on fuel, the Twinstar will beat any other twin. It cruises at 4.5 gal/hr per side.
Id say the Baron and Seneca with the Seneca coming in on top just because it has a lower fuel burn. The Baron is faster though. Both are cheaper to insure then a 300 or 400 series Cessna and the Navajo/Chieftain. The Baron 58 will do 190kts on 26gal/hr where as the Seneca I will do 170kts on 20gal/hr both can seat 6 and fly for 2hrs. If you want efficiency and dont need to fly higher then 10k don't get a turbo model, fuel flows go way up and maintenance is a hassle.