Jonah Hex, for being one of the most memorable of the figures in time that the Legends encounter. Lots of fans are hoping he can become an official member of the Legends.
Future Oliver Queen from "Star City 2046", an Old Superhero from a Bad Future. He was a One-Shot Character in easily one of the best received episodes of Season 1. He was even popular enough to return in the COIE crossover 3 years later.
The Black Flash, a nearly unstoppable undead speedster that pulls a Big Damn Heroes when Thawne and his time remnants threaten the Legends.
Evil Rip Hunter garnered a lot of fans, with his villainous portrayal being very enjoyable.
Axel, Mick's pet rat, if not for any reason simply because he's Mick's pet who the arsonist cares for dearly.
Gary, the Butt-Monkey Time Bureau agent who has his fanboying of the Legends severely tested by having to clean up their crap.
Leo Snart is very warmly received, having all of the humor of his Earth-1 doppelganger but also being a Nice Guy. Helps that, unlike old Snart, he wears his comic basis' costume.
Beebo was so popular that became a meme and ultimately defeated Mallus.
John Constantine, much like brief tenure on Arrow, his time on Legends of Tomorrow made him an instant favorite. Only this time he's sticking around.
Garima was very popular after appearing in Tagumo Attacks, and fans were happy when she appeared in promotional photos for the mid-season finale.
Nora Darhk has become one of the most beloved characters of Season 4. This is mainly because of Courtney Ford's acting, her chemistry with Mona, and her redemption arc is one of the most well-received in the show's history.
Kitty Zari. Zari being transformed into a cat in "Legends of To-Meow-Meow" which fans fell head over heels in love with.
Puppets of Tomorrow. Also from "Legends of To-Meow-Meow" where Sara, Ray, Nate, and Zari get turned into puppets due to Rory being irresponsible with the Fairy Godmother's power.