Mastodon, tbh. You can even follow tags along with users, which I absolutely love.
Edit: Firefish is decent as well, and kinda follows tags with an antenna feature, but it has really only 1 decent app called Milktea and I honestly hate it; so stick to the mobile website for Firefish.
Edit: If you go with Mastodon, I recommend elk.zone as a front-end for it. Much cleaner and easier to use.