Violentmonkey is very actively developed [1]. Greasemonkey isn't [2]. Tampermonkey is closed source so I view it with some suspicion. Otherwise they're all pretty much the same, as far as I know (I don't use them extensively).

[1]: https://github.com/violentmonkey/violentmonkey/graphs/contri...

[2]: https://github.com/greasemonkey/greasemonkey/graphs/contribu...