Not sure if I can think of a favorite, but here are a few candidates:
The fact that Kbin is built for both microblogs and threads. They're neatly distinguished from one another, and the UI accounts for their unique formats. It's very clearly thread aggregation + microblogging, not thread aggregation with microblog posts crammed in there or vice versa.
The UI, which is what made me pick it over Lemmy. I just find it a lot cleaner and more pleasant, especially with the Tokyo night theme.
The community around userscripts and userstyles, or at least what there was initially. It's dwindled a ton, but it's what got me to learn HTML/CSS. Being able to make a userstyle is also really nice for improving Kbin's UI to my tastes.
@ernest. Alongside just being kind, he also keeps us up to date and lets us be involved in the development process. The daily updates on @kbinDevlog have been awesome for seeing and helping the platform develop.