The most popular & time proved RTE's (Rich Text Editors) are without a doubt CKEditor and TinyMCE. However other commercial editors have existed for years and new quality RTE's are popping up (eg. the 2 years old open source & totally free Quill editor). In addition a lot of light to very light weight RTE's are created all the time and many companies are making their own makeshift RTE's for internal use.
The biggest problem with with RTE's is XSS (Cross Site Scripting) - it is generally very easy to post scripts to server side functions that accepts RTE content and it is just as difficult to effectively sanitize such input - actually it is so difficult that programmers are advised not to do it themselves but instead rely on libraries to do it (however at least for ASP.NET Core no such library currently exists).
Below is a list of RTE's worth checking out.