Usagi and Gen are both really popular and tend to steal the show whenever they show up.
The Dark Turtles for their cool designs, personalities, and interesting subplot hinting at the start of a redemption arc in their final episode.
Viral is quite popular among the artist community, due to her unique design and being the lone villainess in Fast Forward.
Chrysalis is probably the most minor member of the Justice Force and never even says a word in the series, but is well-remembered by many.
Traximus, the Triceraton gladiator who becomes Fire-Forged Friends with the heroes and the Rebel Leader against his oppressive government, is seen as a highlight of the three arcs he appears in.
Kirby King, the artist whose drawings come to life, is impressively popular for a One-Shot Character. It helps that he's modeled after the late Jack Kirby.
Eventual Justice Force member Raptarr is the subject of a decent amount of attention, fan art, and Shipping fics due to his appearance and powers, his Ideal Hero nature, and the interesting Back Story of his species.
The Battle Nexus competitor Rings only appears in one episode and is eliminated after a fight that lasts about a minute. Still, his cool use of Deadly Discs and his Barbarian Longhair appearance earn him some fans.
In a meta sense, this version of April is generally better liked than the other versions that aren't Channel 6 reporters.