Disclaimer: As part of the Telerik MVC team (and one of the editor developers), my opinion is quite biased, yet I can shed some light on the differences between the editors.
All of them are open source
Despite Todd's answer, the Telerik MVC editor is open-source, under GPLv2 license (with a commercial license available). Our support is well known, so I guess this is where we stand out (though it is paid). On the other hand, both TinyMCE and CKEditor have huge communities, which may help quickly, too.
Different feature set
This has been the first release of the Telerik MVC editor, while CK and TinyMCE have been around the block for a very long time. We have mixed feelings about this -- while we are missing out on quite a few tools/features, we have invested a lot of time in polishing the existing ones (converting list items to headings, pasting from MS Word). Our bet is on cleaner, more robust code (4k LoC vs 14k LoC in TinyMCE without plug-ins).
It depends on your requirements
After all, with the Telerik suite, you get... well... a suite of components with common code and themes. So if you need the dedicated support and the other components and the limited feature set does not make a difference, Telerik would be a nice choice. If you need a full-scale editor and want to bet on more feature-complete products, TinyMCE and CKEditor are the wiser choice.
I hope this helps and does not contain much happy talk.