As someone also still rocking the original MBPr, the peak for it software wise is actually El Capitan
I really, really wish i had never updated from that. It was faster, smoother, and more stable there than it was before and after. Definitely way better than Mavericks. I push this thing hard with premiere/ableton live/etc too.
My main problem with El Cap is that iPhoto won't work any more. I have a huge investment in iPhoto -- tons of curated albums with no way to import them to Photos (what were they thinking?), and no obvious alternative.
Also, I absolutely despise the El Cap design. The flat look control widgets and icons, the barely visible grey text, the secret hidden controls that only appear when you mouse over them, it all makes me cringe. (Yes, Mav has a few hidden controls as well, but at least now I know where they all are. If I upgrade, I have to go on the secret control easter egg hunt all over again. Life is too short for that kind of bullshit.)
I really, really wish i had never updated from that. It was faster, smoother, and more stable there than it was before and after. Definitely way better than Mavericks. I push this thing hard with premiere/ableton live/etc too.