I know what everyone is talking about with the 240D, it's a very simple and robust design.
I think what kind of alters my thinking about a diesel, and probably a few others in the Northern tier of the US and also Canada, is that a diesel as a daily driver up here can be a bit of a hazard, in terms of needing blended fuel in the winter, good compression, block heaters, etc. Every car up hre needs proper care, but when you look at these additional concerns, kinda dampers your enthusiasm a bit.
If a poll were taken right now though, looks like the 240D would be the winner so far.
A few other honorable mentions I could think of would be the 210 chassis E class, either the V6 or E430 V8, I personally like the E420 210 chassis as well, but for other reasons than just reliability, good car though. Also the V6 engine C280 rarely give problems. Problems are also few and far between for the older C class C220. Only really pattern failure I can think of on the 202 and 210 chassis would be a leaky differential.
MB's equipped with the 104 engines of course often suffer from the wiring harness problem, especialy from the beginning of SFI injection up to 94-95 or so, and the head gasket/front cover leaks, but in general are dependable.