I think depends on what car you really want. It comes down to wanting a full size cruiser or a mid size sport sedan, i'd take the W124 500E or the W126 560SEC so just based on that i'd say the top two evenly are the W124 and the W126...
I'd declare the W126 chassis the best Mercedes-Benz ever made because it is full of positives and almost no negatives. As such, it is as close to "perfection" as Mercedes-Benz has ever come. Every other Benz has one aspect of it or another that people warn you about prior to your purchasing it. With the W126 chassis, such warnings are rare and that's a great feat. The W123s come close, but they have too many issues with rust over time and someday will all eventually disappear. All post 1984 M-Bs were treated for rust-proofing at the factory and as such are much less susceptible to rust. As we all know, 1984 was just in the middle of the production cycle for the W126. The W124 chassis is also very good, but that tiny size is such that I can't even fit in it. Plus, you can't get that indestructible 617.951/952 L5 Turbodiesel engine in it.
Most of the time, for rust to get to those cars, they must have been involved in an accident or something that exposes the rust-proof layer applied from the factory. If the rust proof-layer is placed back, no rusting will occur. If not, as in most cases (most bodyshops don't even know this), rusting starts. MY '84 Euro 500SEL is a great example of one that was in a wreck and started rusting afterwards. My '84 300SD has never been in a wreck, was always driven in salty, winter roads but doesn't even have a spot of rust on it.
One time I parked the car next to a new Porsche Cheyene S and the old guy came out I asked him how he likes his ride. He said, "I like yours better, they are the best car ever made, I had two of them." then he added "they don't build cars like that anymore..."
1986 560SEL Silver/Dark Blue W126
Alpine CVA 1004 with 6.5" display, Alpine DVD navigation system, Parrot CK3100 Bluetooth carkit, European headlights.
Most of the time, for rust to get to those cars, they must have been involved in an accident or something that exposes the rust-proof layer applied from the factory. If the rust proof-layer is placed back, no rusting will occur. If not, as in most cases (most bodyshops don't even know this), rusting starts
Not quite true. The 126 is well know for rust below and around the rear window seal. The rust spreads into the trunk below the rear window too. Not to mention, the bottom of the doors tend to rust around the drain hole, and the fenders are know to rust too.
I'd say the 126 is not even close to being the best Mercedes ever. The best Mercedes ever was most likely the 300 SL gullwing. The R230 SL is an amazing car too.
So how do you explain the fact that I own two rust free W126s that have been in an area where cars rust like hell, like after 12 years car around here turn almost to rust dust as long as they are used in the winter months which both of my W126s were? Furthermore, I've never known about W126s having such a reputation for rust. I beg to differ there!
You can beg to differ but it doesn't make my statements untrue. Do some research. These are known issues. Pretty much every 126 SEC has, at the very least, rust under the rear window. You probably park the car in a garage?
What can I say? You were right, I was wrong... NOT REALLY. Did I just not read carefully, or what it really that no one mentioned in all those search results you posted link to, that rusting was a problem on a W126 sedan? Could it be a problem with the coupes? Perhaps a problem that occurs due to the material used to provide extra support for the rear part of the coupes (due to the absence of the D pillars)? I definitely know what I mean when I say my cars aren't rusting. And yes, my 300SD has always been garaged all its life. But it has also always been driven on salty roads. However, my 500SEL hasn't always been garaged. Results are the same for both vehicles.
I have seen way to many rusty W126's. They also all seem to rust in pretty much the same spots:
Under the rear window, usually hidden by the gasket.
The doors rust around the mount holes for the plastic panals.
Between the front bumper and fender their is a gasket that can hold water and start rust.
The front fender also likes to rust out under the corner light. (thats a strange spot)
Wheel wells tend to get rusty.
And don't forget to check the front sway bar mounts. Pay carefull attention to the passangers side. Battary acid can start some nice rust in their!
I recomend removing and cleaning under the battary tray every few years.
And lets not forget the jack points, but all MB love rusting down their!
They tend to be better then most 80's cars but you still have to be carefull.