I'm most productive on snow leopard.
I use it a lot, It's on my cMP 1,1 that has CS5 installed.
It drives the printers, so a lot of final editing get's done on that machine.
My cMP 3,1 has 64gig ram, USB 3.0 and a GTX 680 graphics card.
It runs on El Capitan. It's one of the main editing machines.
It has CS6 installed
The other main editing machine is a
nMP 3,7 4core, 64gig ram, TB2
It has the Adobe CC installed
I'm thinking on puting high sierra on it.
It is more used as a portable workstation.
( It's the only workstation that is connected to the web,
for security reasons, but also I waste a lot of time, if email is open or FB,
when printing, doing serious editing, color grading, ... )
We also have a 11" Air with Mavericks
and a maxed out 13" Air with Yoshimite on it.
I was dissapointed, that El Capitan just dropped support for our protools hardware.
We don't do a lot of AV. But do have some video gear and modular synthesizers.
We shoot medium format camera's mainly.
And also working with a lot of freelancers.
That could be using anything ... a lot of professionals don't work with the latest and greatest.
They could be using any macbook or macbook pro from 2008 - 2017.
I've never worked on Mavericks, but my partner does a lot of work on it.
I still have a G4 15" powerbook, with Tiger.
It love that keyboard. But It can't even play HD video.
I had a macbook pro but the graphic card got fried.
We also have a alu unibody macbook, it also has snow leopard on it,
it still get's some use. But the airs are just lighter and faster but I guess they will die faster.
I'm most happy on snow leopard and it has the least problems with all the printers.
Epsons A2 and larger.
But I don't really care, even as every new OS feels like a step backwards.
What I miss the most, are all the ports that were on the older macbooks.
The nMP is fine, but the TB enclosures are not cheap at all.