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).