fennec (firefox mobile fork) lets you install ublock origin

newpipe or tubular for youtube. ive also been using freetube lately. its just the desktop ui ported to an android app so its not as good as newpipe, but im using syncthing to sync freetube's settings/subscriptions between desktop and mobile

comaps is better fork of organicmaps and has better people running it

the fossify apps are great for the basics, sms, phone/contacts, gallery, calculator etc

droidify is a nicer front end for the fdroid store imo

keepassdx for passwords

kde connect or localsend for one-off file transfers between devices, or syncthing for things you need to sync all the time

and hacki for hackernews!