I didn't say THE worst, I said ONE of the worst. If I had to give the worst to one chassis it'd be the W201.
The W140 is bloated, unstylish, mechanically challenged, and over-engineered to the nth degree. If you don't believe me, buy one, you'll find out soon enough.
And start saving up now for an evaporator. Scratch that, better save up for 2. Oh, and window motors, vacuum pumps, climate control bellows, switches (all of them), tires wear very quickly, brake pads for stopping that behemoth which leads to rotors and calipers, etc...I could go on.
At least it isn't a W201.