Merc1, I'll reinforce the comment above: There's enough on this topic in the archives of this site to fill at least one book. Spend a few hours (seriously!) and you'll get a lot more than could possibly appear in just one thread.
That said, two insights, both from old posts: 1)A senior engineer at DB was once asked that question, and said the 108 with metal suspension, hands down. That's the 250/280SE/SEL from the latter 60's and earlier 70's, also available in the early 70's with the 4.5-liter V8 (a remarkably bulletproof engine as long as the timing chain service is kept up).
2)Frank Barrett's book, Illustrated Buyer's Guide to Mercedes-Benz. His conclusion is the 124 300E. His words are, approximately, "This is the best buy in a used Mercedes. Period."
Which brings up a point I'd make to you and anyone else wanting to learn more about these great cars: Not only does this site have enough material to fill a few books--there are actually a number of excellent books out there. Spend a few bucks and educate yourself with them. $100 can build you a nice little MB library, and save you many times more than that. Barrett, cited above, is an excellent place to start.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)