I need to develop a plugin for the Content Editor, with a custom tab, commands, and modal windows. Which project template do I need to use? Which .NET version should be used for the project?
sitecore version latest
You can check the following link to get details on the .Net Framework
compatibility with the Sitecore versions.
https://support.sitecore.com/kb?id=kb_article_view&sysparm_article=KB0087164
Since you are working on creating customization for content editor, you can choose Empty Project(.Net Framework)
template with the .Net Framework as 4.8.
Hope this helps!!!