Aldridge would make the biggest difference in Year 1.
Cousins would take some time to integrate and I'd be worried about his attitude, but he's the most talented player listed, by far.
I love Jokic but he's obviously not getting traded, and I don't think he'd make the Celts a lot better right away in any case.
Among the semi-plausible options, I think I'd take Gortat. Rebounds, finishes, and plays D inside.