Now lets start off with what is on Wookiepedia about Ventress.
"However, her general obsession over the Jedi eventually developed into a specific obsession over Obi-Wan Kenobi. It is unknown why she focused on Kenobi to the point of monomania, but his frequent escapes from death at her hands likely had something to do with it."
Then we add the events of the most recent Clone Wars episode where Ventress risked her own life to Save Kenobi from Darth Maul and Darth Maul's brother, Savage Opress. Both of these guys are Sith Warriors in their own right.
Now you may support or dispute my claim if you like.....