From the perspective of the users, Matrix has the "best" experience since it supports bridge controlling user accounts, so all accounts look like real users. Currently the largest part of the community is from Telegram, which is the earliest established platform. Discord comes in second, then Matrix, but there are at least one other contributor is joining from there. Although I want to, the primary goal here isn't to encourage Matrix but to avoid forcing people to use Discord and as ddevault says "set up users who are passionate about free software — i.e. your most passionate contributors or potential contributors — as second-class citizens".