I use Alexandrite on desktop but I'm excited about Lemmy-UI-Next which (IIRC) is being developed by one of admins of lemm.ee. It looks very promising.
For mobile I'm using Voyager (this account is actually on the instance for supporters.) I've tried everything available for iOS but Voyager is my daily driver, nothing else comes close to ticking as many boxes as Voyager does for me. I have Apollo sideloaded on my phone and it's incredible how much progress Voyager has made toward replicating the experience which is exactly what I want in a mobile app for Lemmy. The only thing it's missing is my favorite Apollo theme (Outrun).
Photon on desktop (although it's excellent on mobile too), however I use Boost on Android, and Voyager on iOS. Native apps always feel a little more cohesive
Arctic is my primary, it was the first that I know of to include admin tools, the single developer is super accessible and on top of for resolving bugs and does a good job at supporting markdown both in the editor and the viewer.
Mlem is my backup if I need to access 2 different spots on the Lemmy-verse. It has a great viewer and interface but is missing a markdown editor.