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".
It doesn’t, you aren’t compelled to use Signal, I care that the clients I’m talking to are conform to the spec and don’t expose myself to side channel attacks (e.g. previewing content directly from the client)
Signal is not alternative to Messenger. Signal is an alternative to whatsapp.