Kup is easily the weakest SS I've experienced. He's not really animation model accurate, I think he's too scrawny. I wasn't going to bother, as the TR seemed the better option, but the rest of SS won me over.
I do, however, think he's better than TR, but Legends...? Depends what you want from a Kup. He's got a better vehicle mode, but it's a lot more fiddly to get there.
I don't have the Legends one for a proper comparison, but I suspect he's probably a tad more fun than the SS. It's super close though, he's growing on me as I get used to him.
There's an odd trend with Kup though. Last toy you could pull his head off, this one you can pull all his other limbs off!
I decided to ditch my TR Kup at some point in the future. SS looks better, even if it sounds fiddly as hell. Ive still got the Orion Pax version of the mould which will do for me. Never noticed the changes to LG Kup though, but then I only realised how improved Hot Rod was after getting the LG one a few months back.
The Generations mould is still a favourite. Ive got 6 different versions of it I think.