I'd say Telegram is probably the "nicest" app I've used for messaging. WhatsApp and Messenger are Ok. Signal, whilst being idealistically the best, is spartan, kinda ugly, and lacks many features that make messaging "fun".
Telegram has been picking up steam in my wide network. It's been surprising to me how easy it was, as an early adopter, to convince people to use it. It doesn't work with everyone; WhatsApp dominates around here. But in the tens of people that I care to communicate with have happily switched to Telegram for much of their chatting.