As long as Maul utilizes Tk I give him the win. Most higher tier Jedi/Sith should be able to defeat Grievous with extensive use of Tk at least. But If it was just Sabers though then I'd say split.
I don't remember anyone outright defeating GG in a pure Saber match up except for Ventress on Dathomir and Kenobi in ROTS. Kit Fisto did well against him but he also resorted to Tk Iirc and still never actually won.