If you prefer an open source app, native, complete, with a decent UI and an active developer community, the official one is definitely the best. And I mean Jerboa, because it is the official client made by the very same people that are making Lemmy (so always compatible and up-to-date with the server development).
Otherwise, if you don't mind using a closed source app, Summit is the most feature rich one and it is maintained by a professional industry-level native Android developer who really knows what he's doing.
Finally, if you want to experiment with a cross platform app, open source and well maintained, with a really nice user interface, have a try with Thunder.
It is nice but development seems to finally have stopped. That said, right now it works perfectly fine. Just once Lemmy servers undergo major changes it might not anymore.
Jerboa, IMO. The UI is acceptable. There aren't many bugs. It's updated with good regularity. Because it's the most common, it's also the most supported. It may not be exciting, but it's pretty set-and-forget as far as configuration and troubleshooting goes.