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.
'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)