I have used most of the messengers on f-droid.
Delta-chat is really cool, but, inconvenient if the other user doesn’t have the app. Anyways, it fills your inbox with too many short mails. Also, it is not for instantaneous messaging due to email delay.
Riot and telegram are popular messengers too.
Kontalk and Conversations are nice.
Kontalk is phone number based, which is an advantage for some users. Using kontalk with other xmpp servers is never encrypted.
So, universality is missing.
Conversations solves this.
A messenger which needs special mention is Pix-art.
I am impressed with pix-art messenger. It is based on conversations, but more user friendly. It also suggests some pretty fast public xmpp servers and one can register with them on the app.