I think the early '80s were the sweet spot. they have a lot of the improvements but before you ran into circlip issues or paralevers.
I've only owned an r80 but I've ridden many others and I have to say I like the 80 the best. Below that and you get a little less power, above that and you get vibration and the gas mileage seems to go down.
I like the monolever final drive but they're way overpriced right now. The twin shocks work the same and they look classier. The ST tanks don't do it for me, the sidecovers look cheap and that plastic airbox ruins the look of the engine.
The Nikasil cylinders are famous for lasting forever. I consider them a big bonus.
I've heard many many people describe the ST as being the 'best handling air-cooled BMW' every made. I took one through the mountains a while back and I was impressed. I'd still take the /7 based on looks.
All that to say 'yeah, I agree with the other guy.' Put my money on an early '80s r80/7 or r80ST