I believe the big body w126 probably has the most "tanklike" feeling..(being the largest and heaviest).
You can open and close the doors of a w123, a w126, and a w124 and I think you will feel the solid shut and the most heavyness from a w126.

I have tinkered with all these cars and own both a w123 and w126 yet the w126 just feels more tanklike...the w123 feels a bit more sporty.