Captain Cold might have gathered mixed opinions over his Adaptational Villainy, but the character himself is an undisputed fan favorite. He is regarded as an excellent nemesis to Barry, being a Magnificent Bastard, with his episodes being among the best. He carries with him a degree of sophistication and finesse to his villainy, with his lines being very theatrical and delivered brilliantly by Wentworth Miller. Considering his popularity, it's no wonder he ended up as a main character on Legends of Tomorrow.
Linda Park has become one before she even appeared, simply because it was announced she was being played by Malese Jow. Technically she's The Other Darrin because Linda has had a brief cameo played by another actress, but still. She ended up proving popular enough some fans were hoping she'd avoid being a throw-away temporary love interest, leading to excitement when it was announced she'd return in Season 2, and excitement when Jow expressed interest in returning for the third. Even more so when she was revealed to be a metahuman on Earth-2, leaving some fans hoping the original gets powers as well.
Gorilla Grodd. His first appearance was one of the best scenes in the series up to that point. The buildup to a full appearance by Grodd has been driving the fandom absolutely crazy.
Abra Kadabra. One of the few Rogues who is adapted faithfully, is a proper formidable opponent, and isn't treated as a throwaway Villain of the Week. Despite having a single episode appearance in Season Three, he was one of the most beloved characters to appear there. His return in Season Seven, giving him a sympathetic backstory and a HeelFace Turn only sought to win over even more support from fans.
King Shark proved extremely popular and memorable despite only appearing for 45 seconds. Positive fan response even led the creators to bring him back as the main villain in a self-titled episode later in the season. He eventually got to make a full HeelFace Turn down the line!
Patty Spivot. The one temporary love interest that the fandom affectionately remembers. Beloved for how kind and accepting she was too Barry, while also having a badass side. Many thought she could have been a great addition to Tean Flash and that Barry turning her away was wasted potential.
Earth-2 Iris is also very popular in the fandom. Many see her as much more badass than her Earth-1 counterpart, a given as unlike the original Iris, this Iris went through the police academy.
The man in the iron mask in "Escape from Earth-2" has gained a lot of interest for his mysterious background.
Out of Zoom's Earth-2 minions, Killer Frost and Black Siren (for obviousreasons) became instantly adored by the fanbase.
Jay Garrick due to becoming a mentor to Barry and finally talking some sense into him, and for being just a Cool Old Guy in general.
Out of all the alternate versions of Harrison Wells, Mime Wells and Hells Wells were the ones who endeared the fandom the most.
The Accelerated Man a.k.a. The Flash of Earth-19. His brief appearance accompanied by his very old fashioned but memorable outfit has made him a One-Scene Wonder and left many fans interested in what his story is. Plus his lightning appears to be purple!
The mystery girl, played by Jessica Parker Kennedy, manages to avoid the show's usual Arc Fatigue of introducing characters without disclosing their identity immediately, likely due to the restraint shown on limiting her appearances and her tendency to steal her scenes with her adorable, bubbly personality. Helps that she's strongly implied to be Barry's and Iris's daughter from the future. Her identity is confirmed in the season finale, although her name is Nora instead.
Chester P. Runk. Much like Team Flash In-Universe, Chester is very easy to like given his Totally Radical, Keet, Nice Guy personality, as well as an enthusiastic online streamer. Not to mention he is a 100% sympathetic meta with the endangering lives he caused being a complete accident.