In Canon? Ventress. And it's not even close. Grievous is a bad joke in canon.

In Legends? Grievous. But it's way more close than in canon.

^My thoughts exactly. Canon Grievous is so poorly written compared to legends Grievous. How can even call himself a Jedi killer in canon when he can’t kill a rookie padawan and some gungans?

Canon: Neither I don't care for what happend to either of them in TCW.

Legends : Actually im gonna go with ventress if there was a both option I'd choose that because the more I learn about her in the EU the more I have begun to love her character and her progression to actually becoming a good person, I also love how she managed to escape after she faked her death by Anakin. Plus she is so skilled in the EU which is portrayed thorough the many Republic comics and 2003 clone wars. I love grevious as-well I love how he was almost unbeatable and how he was just a walking meance that people stood no chance against. I love how he was potrayed in 2003 clone wars and in Republic comics.

Legends grievious canon ventress

I personally like General Grievous

But I say that Asajj Ventress was cool too