I would say a 108 chassis 280 SE (with the straight six) or 108 chassis 280 SE 4.5 (4.5 liter V8).
They are pre-smog era car, no catalytic converters, basic mechanical fuel injection with a distributor cap and rotor and no ECU, EHA, ABS, or any other computer controlled acronym.
Plus, they are not only shamefully undervalued (beater cars can be bought for $2k-$3k and wonderful examples for $5k - $7k), but they are alos considered by those at Stuttgart to be the best Mercedes ever built. Refer to this article in the Star Magazine:
Comments on the 108 chassis in The Star Magazine 1997 issue