Well, my two Ringgits worth:-
1) For sheer longevity and durability, the W123 series. I had a '78 W123 300D as an 18-year old way back in '82 when I started college in California. Sold it to someone in Fresno and the car is still cruisin' up and down CA Highway 99 regularly. Here in Malaysia, the W123 diesels make the bulk of our "interstate" taxi fleet. The 200D, 240D and 300D taxis abound on our roads and highways. From what I've heard from the battle-hardened taxi drivers, the W123 240D (manual) is, indeed, the toughest thing ever built. Well, these things come from a long lineage of shock-proof predecessors. You can still see many W114/115 and even W110 diesels used a taxicabs.
2) For overall practicality, durability, utility, and built quality, the W124 series. These machines can be found in the thousands on East Asian roads. The W124s are really the last of the non-plasticky Benzes.
3) For sheer emotional appeal and character, the W116 series (the original Euro model of course). Remember Ronin? When the W126 came out (especially the early models), I thought the Daimler-Benz people must have gone nuts. How could the sheer class and classical form of the W116 S-Class be replaced after a short 7-year model run. Anyway, I have a '78 W116 280SE in pristine condition. This two-ton gracefully athletic behemoth, propelled by the sweet-sounding 185hp M-110 DOHC I-6 motor, is poetry in motion. My 23-year old W116 is (to use someone elses phrase) a better "bxxxh magnet" than my E36 BMW 325is.
The honors list: W123, W124 and W116.
Honorable mention: W126 and W107.
1978 W116 280SE Euro
1995 E36 BMW 325is
1984 Volvo 240GLM