Even though I own a W116 300SD, I'd have to say the W126. Don't get me wrong I love the fact that my big old 116 gets the fuel economy of my smaller VW, but as with many 116s, rust can be a problem.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section