The best years were 1994-2003 with the 7.3 liter Powerstroke. These engines were the most reliable. After the 7.3, the 6.0 Powerstroke was introduced. It had more power but was a lot less reliable, as many consumers had problems. The 6.0 was discontinued in 2007 when the 6.4 liter Powerstroke was introduced mainly due to govt. regulations on diesel emissions. This engine uses the same block as the 6.0 but is more powerful. It has fewer parts and is said to be better than the 6.0, but not better than the 7.3. The 6.7 liter Powerstroke is supposed to be out this month. It has more stock power than any of these previous motors mentioned. Furthermore, it is now made in-house by Ford, not Navistar like the previous engines. The cylinder heads are also aluminum on this engine like the Duramax used by GM. Time will tell how reliable it is.
If it were me, I would go with a 2007 6.0 Powerstroke. The engine had most of the bugs worked out at this time and it does not contain all of the emission controls like the two newer motors. I also think the trucks from 2005-2007 looked the best. If you go with the 7.3 you are likely to only find trucks with high miles.