I'm really impressed with Fedilab for Android and its open source. https://f-droid.org/en/packages/fr.gouv.etalab.mastodon/ its also on the play store if you don't use F-Droid.
I can't remember what I liked on iOS, I think Tusk was one of the good ones.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko