One with a bunch of cursors editing the same text would be great.
As of now found:
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Web-based there isn’t AFAIK a good one for code editing. There used to be a good one based on Etherpad, but it has been abandoned some years ago.
I personally like: https://www.saros-project.org/ Not web-based, but all you need is a somewhat modern XMPP server to do the communication, no other server side component required.