From what I know, it's a mix of Damus, Nostter.app , Rabbit & Amethyst. Old Iris also gets a lot of love 💜 New users tend to be from Damus/Nostter mostly. (Not sure if Primal, Nostur, Snort, Coracle, Nostrudel is used much 🤷‍♀️)

I love Rabbit. https://rabbit.syusui.net/ or https://nostter.app/