Off the cuff, I’d say it’s tied with Summit and How We Feel.

If you don’t know, Summit is a lemmy and piefed app. It gives me the same feel of customizing as Sync for Lemmy, but with way more attentive and present dev.

As for How We Feel, it’s a mood tracker that’s supposedly backed by a university. It has videos and steps to generally manage my emotions (also read them if I don’t want to watch them). The best part for me is that the app have descriptions for their list of emotions. Really helps me a lot when I’m just overwhelmed OR lost for words to describe my emotions. The act of looking for the right emotiol itself is IMO grounding.