Like there is the Official Mastodon app for Mastodon, and Element app for Matrix, which app is considered the most official lemmy app for Android [possibly also for ios]
Or atleast, the app which supports most(if not all) the features available in Lemmy
That’s very subjective, so it’s hard to say! I find Sync to be the best, but I enjoy the look and feel of Thunder.
I’d say give them a whirl, they’re all free and most run without signing in just fine