Taking a look at the best returning guest for DC's Legends of Tomorrow, there are a few interesting candidates like John Constantine (Matt Ryan), Nora Darhk (Courtney Ford), Kuasa (Tracy Ifeachor) and Jonah Hex (Johnathan Schaech). Most of them are basically eliminated though because they are returning but not from this series. And Hex would've won and should pop up in every season at least once, but for the return of Wentworth Miller as Citizen Cold.
This may be a bit of a cheat because Miller originally played the Captain Cold, aka Leonard Snart, first on The Flash and then in the first season of Legends. He then returned in season 2 as a version of Snart before he met the Legends and was still a bad guy. He was part of the Legion of Doom near the end of the season.
But this year he was brought into the crossover, Crisis on Earth-X, as Citizen Cold, the Earth-X version of Snart that is actually a hero and is well connected to his emotions and feelings. When the battle against the Nazis was over, Citizen Cold decided to stick around with the Legends for a little while to help them cope with the loss of Martin Stein (Victor Garber).
As much as I liked Captain Cold and his cynicism, Citizen Cold is a lot of fun and was able to bring some great moments to the series whether he was prodding Mick Rory (Dominic Purcell), using a puppet of Stein to get them to talk, or turning down an advance by Constantine. Miller's unique delivery and portrayal of the character was one of the highlights of the season. It was great to see him pop up on The Flash as well — I mean, someone had to make a joke about the Elongated Man's name… and I'm hoping the writers find more ways to bring him into the series again.