Well i have to say It's a tie between my Duna Tripper 6 and Cruiser Mk VII. Both date back to 17.0 and are super reliable. Tripper 6 is a Duna capable resupply ship. Carries Three men. There have been 15 flights to Duna, all have returned safely. Can carry a 5-10 ton payload above the cockpit. The Cruisers Mk VII, the latest in a long line of Cruisers, is a long range orbiter capable of some of the farthest reaches of the Kerbal System. It can carry a 4-10 ton payload up with it and still reach Laythe. 30+ flights have been made in it, including the first (17.0) landing and return to Duna soil. 19.1 flights include the "Dragon," to Duna and Ike, the "Claudia," to Laythe, the "Riley," to Eve and Gilly, and (pending launch) the "Guyra," to Dres.