My order of who I want back...
Leonard Snart
Martin Stein
Laurel Lance (Earth-1)
Oliver Queen
And I mean Leonard, not Leo. Aside from the best Cold shades, I didn't feel Leo brought anything that made him better than the Earth-1 Leonard.
Would've put Oliver above Laurel but then I thought what would happen if the Canaries spinoff doesn't happen. Of course I think it is happening but presuming if not. They could bring Laurel in as a Legend, should they choose to, and keep Katie. However, it would also ruin season 2's great storyline where Sara moves on.