>What about iOS?

Siskin [1] seems to be the best client for iOS right now. It's not as good as Conversations.

>What about desktop?

I am pretty happy with beagle (OSX) [2], Dino (Linux) [3] and Gajim (linux, windows, OSX) [4]. If in-browser is your thing, converse.js [5] or movim [6] come to mind.

>What if I don't want to run my own XMPP server?

Use quicksy.im?

[1] https://siskin.im

[2] https://beagle.im

[3] https://dino.im

[4] https://gajim.org

[5] https://conversejs.org/

[6] https://movim.eu/