lichess: non-profit, free and open source, community oriented, high quality tech, growing userbase, amazing founder ornicar, much loved by its users and supporters including me. Regular online competitions well attended but doesn't organise major events. Doesn't really do things like video lessons or partnership with streamers as far as I know, probably due to non-profit status and community focus.

chess.com: for-profit, the largest user base, led by IM Danny Rensch. Tech is not as good as lichess but still okay-ish. Organises major events with GM commentators. Has partnerships with major streamers, such as the most successful (but controversial) US number 2 GM Hikaru Nakamura. I used to support them before lichess got big.

chess24: for-profit, led by GM Jan Gustaffson, much smaller than chess.com and lichess, supported by current world champion GM Magnus Carlsen. Tech is worse than both lichess and chess.com. Also organises important events, and also produces high quality content such as video lessons. I'm considering supporting them too as I watch a lot of their content but I just enjoy playing on lichess too much.

One fascinating thing is how both for-profit chess.com and chess24 have much worse tech than non-profit lichess despite presumably large budgets. For example chess24 platform has been under development for years and has become a running joke even for its employees. You'd think a chess website would be fairly simple tech in the grand scheme of things but clearly it's still hard to do it right. A testament to how impressive lichess founder and community is.