You can't beat the W123 diesels for reliability - and as an added bonus they are very easy to fix. My favorite pre-90's Diesel is a W123 240D with a 4 speed stick. Lots of room under the hood and a simple manual HVAC system.
For post 90's the 300D 2.5 turbo is a superb car. Great power, reliable and fuel economy in the 33-36 MPG range. Only downside here is these cars are much more costly to repair - they have lots of electronic gizmos that will fail and are not repairable.
As Larry points out beauty is in the eye of the beholder but IMHO if the OM606 4-valve turbo engines prove reliable over the long haul the hands-down-no-brainer choice for best MB Diesel of all time will be the W210 body E300 turbodiesel....Unless we ever get the CDI engines on this side of the Atlantic.