I really like simple design of Telegram. Also all the animations etc. are very smooth. 

 imo discord used to be nice, before it became so overburdened with all kinds of shiny advertised click-here pay-here gamer show-off feature nonsense
it's a mess now

signal is my favorite now, it's simple and straightforward, it works and is secure, and doesn't have any significant frustrations for me 

 Google Messages for Android. Simple and elegant. I'm not familiar with current iOS though.