What is the most rare w116? Your suggestions...
By raw numbers the 350SEL is the least produced version, followed by the 280SEL and the 450SEL 6.9.
As Beastie said, the economics of the market would probably make the 280SEL the most rare today - an entry level model, which probably not many in its target market cared much for long-term, and with little re-sale value in its time.
In the year and a bit I have been watching the UK, Irish and German markets very closely, I have only seen one 280SEL for sale, and two 350SEL. Six-nines are far more frequent, yet far more expensive as well (as they're considered a collectors item, unlike the other two).