Going on for as long as it has and with Loads and Loads of Characters, it's no surprise that the fans would have some favorites.
Note: Because describing things would be hard otherwise, all spoilers are unmarked. Please tread with caution.
- Blue Diamond's Pearl gained a loving fanbase and a lot of fanart following her surprise appearance in "The Answer", due to her silent nature, cute design, and being a complete contrast to the Crystal Gem Pearl.
- Yellow Diamond's Pearl, much like her blue counterpart, garnered a lot of fanart and a big fanbase thanks to her hilariously sassy and smug attitude, and again for being a complete contrast to both CG Pearl and Blue Pearl.
- Centipeetle was in demand for another appearance after her reintroduction in "Monster Buddies". Her endearingly gross design and adorable dog-like behavior, coupled with her touching sacrifice to protect Steven won her many fans. Helped along is that she was the first tragic element depicted onscreen that couldn't be resolved at the end of the episode, establishing the first of the show's darker storylines. Her popularity further increased after "Monster Reunion", which showcased her partially uncorrupted form and her tragic backstory. Many were somewhat disappointed that she didn't appear after "Change Your Mind", despite getting a redesign.
- While the Famethyst in general are loved by many, Skinny and Carnelian, being the most stand-out members of the Beta Kindergarten Gems (besides Jasper), are usually given more spotlight in fanwork than their cubby-mates. 8XJ is also highly popular among the Amethysts of the group.
- The snobby, arrogant and sadistic Aquamarine became a fan favorite after her episodes were first seen in full, simply due to how blatantly, entertainingly bitchy she is, and due to her not being, in any capacity, able to be reasoned with. The end of "Stuck Together" potentially hinting at her having a soft side in spite of this helped matters too.
- Blue/Defense Zircon quickly gained a following for her awkwardness and exasperation, but for also being the first character in the entire series to notice and reveal the inconsistencies in Pink Diamond's shattering. Being voiced by comedy goddess Amy Sedaris doesn't hurt either.
- The titular "Off Colors" debuting in the episode of the same name were all rather well met by fans, but the Adorkable Padparadscha in particular immediately became quite popular after a two-minute clip from Lars' Head featuring one brief line from her was leaked, and drew a huge amount of adoration from fans when the episodes actually aired, thanks to her cute design coupled with her endearing and entertaining quirk and personality. Besides her, Fluorite also stands out somewhat in terms of popularity, not only for her sweet demeanor (with many fans calling her "Grandma Caterpillar"), but she also gained a lot of appreciation for being a very rare (not to mention adorable) instance of positive representation of polyamory in a children's show and in general.
- Like Blue Pearl and Yellow Pearl before her, White Diamond's Pearl/Pink Diamond's Original Pearl quickly became popular after her first appearance in "Legs From Here to Homeworld" thanks to her missing eye and odd nature. Her popularity levels only increased in "Together Alone" and "Change Your Mind", which revealed her to actually be Pink's original Pearl, who White Diamond took over (quite literally) after Pink misbehaved one too many times. Many immediately felt sympathy for her and quite a bit of fanart and fic were made where she was freed of White's control and/or sent to Earth where she could heal.
- The Pebbles, for being adorable and the only characters besides Pearl who consistently stick with Steven when he's alone on Homeworld.
- Comby also gained some recognition, despite her little amount of screentime. The concept of a Gem being used for something as insignificant as a comb on Homeworld, when Gems wouldn't even need a hairbrush, makes her a prime source for Epileptic Trees.
- Many of the uncorrupted gems seen at the end of "Change Your Mind" are getting lots of love, to the point where many fans wish that Season 6 focuses on exploring all of these long-hidden personalities. Steven Universe: Future did establish some of them, but there is still quite a lot of room to fill in the blanks.
- Particularly the three former Crystal Gems, Biggs, Crazy Lace and Snowflake already got a lot of love from being first mentioned in "Bismuth". With all of them getting officially revealed in "Change Your Mind", their popularity has skyrocketed.
- Little Larimar is by far the most popular of the more established "Little Homeworld" gems, due to her Creepy Cute appearance and her bizarre fascination with screams.
- The Geode Beetles of Heaven and Earth were fairly popular Gem Monsters, the Heaven Beetle's sentience even alluded to future developments about the corrupt Gems. When finally uncorrupted, their adorable Gem forms received a lot of interest and fanart.
- Orange Spodumene didn't make a proper appearance outside of cameos in the series, but many fans pay a lot of attention to her due to her somewhat cute design.
- The pair of Lapis Lazulis from "Why so Blue?" received tons of attention when the opening for Steven Universe Future was first revealed and even more so when they actually appeared, the Lapis with golden freckles (dubbed both Nice Lapis by the ending credits and Freckles by the fans) especially for her cute appearance and Adorkable personality.
- Aubergine Pearl is practically just background decoration, as she merely works as a tour guide and has only 10 seconds of actual screentime. However, due her deep voice and unique design, as well as being the only Pearl who didn't either serve a Diamond, isn't part of a permafusion and actually appeared in the show itself, she quickly gained a fanbase.
- Flint and Chert are also just bit characters, but are quite popular in the fanbase for their fun designs and cute relationship, but especially for being Development Gag characters, who finally found their way into the series. Even back when they only had concept art, they were quite adored for being interesting characters, as they were Quartzes who hated fighting.
Gems (non-canon and Meta)
- Quite a few Level 2 canon Gems have gotten quite a lot of attention, to the point of fans actively wishing for them to appear in the show, even if their existence falls under Schrödinger's Canon.
- Save The Light gives us Squaridot, a Replacement Flat Character of Peridot who's loved by those who missed Peridot's pre-HeelFace Turn personality and consider her a nice compromise, and wish she had more screen time.
- The Phantom Fable:
- Lonely Pearl won quite a few fans over with her design, Tear Jerker backstory and her possible corrupted form. Her being the first non-Diamond, non-Fused Pearl introduced also helps her popularity.
- The Mooks in the Amethyst area, who many fans have dubbed "Ice Quartzes", gained quite a lot of attention despite not being plot-relevant at all, for the novelty of being a new Quartz type and their interesting weapon choice.
- Unleash The Light has minor Gem characters who are forced to work under Demantoid and Pyrope. All of them have gotten some degree of attention, but none so much as the first Peridot, who is called "Peri Pal". Her unique hairstyle and her Desperately Looking for a Purpose in Life made her stick out somewhat, though the two Jades (one blue and one red, not Lemon Jade) get similar attention.
- Out of all the beta Gems from the Art & Origins book (most of which already have a small share of fanart) Budgerite has to be the most popular of the bunch, simply because she's a Nonhuman Humanoid Hybrid, being part-Gem, part-bird. While the book doesn't specify how she came to be, the idea of another hybrid besides Steven fascinated many fans, especially since she's half-bird, meaning that Steven can keep his status as the only Half-Human Hybrid while still exploring the concept of other hybrids.
- While all the pilot versions of the Gems received fanart, Pilot Pearl remained the most consistently popular for her unique design and vastly different personality to show Pearl. Some even wish for her to be made into a separate character and maybe even interacting with her main series counterpart.
- Opal is one of the most popular Fusions, quite impressive considering she's only had three actual lines in the entire series. "Hopal for Opal!" is a common cry in the fandom, and whenever she does make a cameo, it gets a good number of cheers — her sighing in "Log Date 7 15 2" was one of the most popular moments in an already popular episode.
- Sardonyx became overwhelmingly popular practically overnight, due to her Large Ham personality and being the long-awaited Fusion of Garnet and Pearl, and to this day those who love her far outweigh her detractors.
- The love for Topaz(es), skyrocketed once she spoke in "Stuck Together", revealing her to be incredibly sympathetic to Steven and Lars' plight and a lot more emotional and distressed at following orders than one would initially believe, with many fans immediately wishing to see her join the Crystal Gems in the future. Her distinctly feminine voice itself was not at all what most fans expected, but was received very positively and seen as fitting for her character nonetheless.
- The two Jades in "Together Alone", along with their Fusion, Lemon Jade, immediately garnered a following for being adorable, being another cross-Gem Fusion, and being completely willing to oust themselves and side with the then all-Fused Crystal Gems.
- Rainbow Quartz 2.0, anticipated since "We Need to Talk", finally makes his debut in "Change Your Mind", and the fandom practically exploded due to him being a new spin on the original Rainbow Quartz, and essentially being the male Fusion version of Mary Poppins.
- Sunstone is also beloved by a good chunk of the fandom, due to their interesting design and well-intentioned, if patronizing, personality. Being the long-awaited Fusion of Steven and Garnet helps greatly.
- In spite of only appearing for about two minutes, Mega Pearl quickly became a fandom darling, for the dashing design, cool personality and fighting style and being a long awaited fusion between two Pearls, especially since it's a fusion representing a healthy, mutually beneficial relationship.
- Bluebird Azurite was already quite popular when she only appeared in the intro, since she was clearly a fusion of Aquamarine and Eyeball, two fan-favorite villains. After her episode aired, she gained a lot of love (and also a lot of hate), especially due to her somewhat hilarious origin. Being the first villainous fusion since Season 3 helps a lot.
- Sadie Miller is one of the few Beach City cast that doesn't get a divisive response. Being perky, helpful to Steven, occasionally getting in some of the action as the only human sans Connie to defeat a corrupted Gem on her own, and singing what's become a hit song with the fanbase with her own shyness and insecurities. Even her crush on Lars back when he was quite polarizing ultimately doesn't detract from this.
- Vidalia appears for barely half a minute in "Story for Steven" and only had a short line, yet she got quite a bit of fanart from people fond of her "80's bad girl" design, and spawned many theories about her being Sour Cream and Onion's mother◊, due to their designs being similar and the Theme Naming between the two. When "Onion Friend" revealed this to be the case, in addition to her being a laid back, loving mom that completely supports Amethyst when she vents to her, she gained even more fans.
- Peedee Fryman became quite popular after his debut in "Frybo", and many fans agree that he should have more episodes focused on him. His cynical realism when juxtaposed to Steven's upbeat optimism makes for an interesting dynamic. Not to mention his disillusioned commentary on having a job really hit home for a lot of fans, who in return want him to be happy. Plus, he's also just downright adorable.
- Mystery Girl, aka "S". Despite not having any audible lines in her debut, this punk rocker who bears a distinct resemblance to Rose instantly became a favorite with the fans, with Pearl's obvious crush on her leaving the homosexual subtext even less room to hide than usual, and her whole design and likeness to Rose being fascinating.
- Shep, the first fully non-binary character in the series has gotten a lot of love, despite being a relatively minor character in the grand scheme of things. Especially with their pleasant personality and somewhat wholesome relationship with Sadie.