The all time best car that MB ever made is probably the W126 chassis, known in the US as the 300SD, 300SDL, 350 SDL, 300SE, 380 SE/L, 420 SE/L, 500 SE/L, and 560 SE/L. I'm not sure which variants were available in both long and short bodies, if any. Some may be the long version in some production years and not others.
All are VERY long lived, and can easily see 300,000 miles without much beyond routine maintenance, remembering that MB are HIGH maintenance cars, unlike Honda, Toyota, etc.
The run-forever champion is probably the 300 SD with the 617 turbodiesel. These can run 500,000 without more than a valve job or two. All the rest have aluminum engine parts with higher failure rates. However, I have a 300D with nearly 200,000 miles on it without major engine work (may have a new head though). My brother has an 86 300SDL getting close to 200,000 without any work at all beyond regular maintenance.
For the gas engines, the 300 will be kinda slow, but gets better milage (20-22mpg). The 380 and 420 V8's are known for timing chain failures -- the best value in used MBs is probably a 420 SEL with a new chain the owner wants to dump. The 500 and 560 are bulletproof and fast, but hungry -- can get as low as 12 mpg if you do lots of stop and go driving.
The W124 chassis is nice, don't know all the numbers over the years, but hte 87 300D is a rocket by diesel standards, and the later 300D 2.5L is nice, too. 300E models are common, and can usually be restored to new condition if you want to part with the cash.
The W123 chassis is nice, but finding a rust free one is getting difficult. (80-85 300D, 240D, 280, 300CD, 280C -- I think).
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!