Unlike most, if not other, Steven Universe villains, Spinel is the most fleshed-out and three-dimensional one in my opinion. Peridot might be "Best Gem", but Spinel is "Best Villain".

Here are some reasons why I think this about Spinel (Spoilers):

1. Spinel has an appealing and unique character design that makes her stand out from the rest without looking too out-of-place. She's designed in the style of rubber-hose animation (which also contributes to the other parts of her character). In her innocent form, she's more cute and adorable, as well as similar to Mickey/Minnie Mouse in terms of design. In her insane design, she's more creepy and deranged, as well as a more villainous, pink gem version of Jenny XJ-9 from My Life as a Teenage Robot. Both are simple, yet effective at conveying her personality and tie very well into her backstory (which I'll get to later).
2. Her personality is very expressive, hammy, and over-the-top, both in her good and evil forms, but that doesn't undermine her other traits. She's a mix of goofy, tragic, adorable, and threatening. All great combinations for a villain like her. This makes sense as she was literally created to be a court jester and best friend to Pink Diamond before her descent into villainy and insanity.
3. Her animation and songs are both very well-made. From the former applying the 12 principles of animation to such a glamorous degree, akin to a Disney film, to the latter being well-sung and gives us a lot of context into her character.

4. And last, but not least is her backstory and character/redemption arc. Unlike most villains in the series, especially the Diamonds (Sorry for bringing that up), Spinel's backstory is legitimately heartbreaking and has a lot of depth to it. She was supposed to be Pink Diamond's best friend, playmate, and jester in order to make her happy, but Pink Diamond abandoned her and left her alone in a garden in space for 6,000 years. Spinel waited for Pink to come back for her for that long under the lie that they were playing a game, but after Steven's message came into the garden, informing her about Pink's death and her moving on without her and forgetting and replacing her, Spinel finally snapped under her repressed insanity, self-poofed and reformed, and decided to get revenge on Pink Diamond by taking away everything that took her away from her.

   Her character arc is very well-written, as it's clear that she doesn't want to get revenge or hurt others, deep down, but feels as though she needs to in order to heal her pain and trauma. She even realizes how horrible she's become and that she can't change or improve and will only get worse no matter what. Even after her full-redemption after calming down, she still thinks that she's caused too much trouble to be forgiven and doesn't deserve Steven and his friends' forgiveness after all the pain she put them through. However, she was given another chance to start over with the Diamonds and now has the loving, caring support group she needed for so long.

And that's only most of the reasons why I love Spinel. A villain with true nuance and depth that we actually wanted to see redeemed. What do you guys think of Spinel? Do you agree with the points I made about her or do you disagree? Let me know by either replying or via PM. Thank you and have a great day.