What fonts can I safely remove?

That depends on which apps you are using. For making it easier any fonts which are not part of your System, like User fonts or any other Collection you'd created. The safest way will be to “Disable” fonts instead of ‘Deleting”. That way if a particular app request a font you will have it and can be re-enabled.

If you are more confident, in Font Book go to Edit menu » Restore Standard Fonts.

There are a lot of them most users don't need. Nor does the OS itself. See my article, Font Management in OS X.

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