The 15 episode of TCW chronology. While Grievous claims that his "upgrades" were voluntary, in Legends (and passingly implied in canon) it was a self-delusion/attempted rationalization. I've watched the 2003 Clone Wars miniseries, and while I'm disappointed with how Grievous (and everyone else) got nerfed,(there's the fan-theory that the miniseries was an actual in-universe propaganda piece (exaggerating everyone into OP territory does make some sense)), I still enjoy TCW cartoon because it's more "realistic" and closer in comparison to the movies.

The finale of TCW begins. At long last, the (in)famous Siege of Mandalore begins in the twilight moments of the Clone Wars. (The episode is concurrent with just before the start of Episode III: Revenge of the Sith)

The 50 episode of TCW cartoon chronology. The Trade Federation and Pantora are at an impass, leaving both sides attempting to sway events in their favor.

The 100 episode of TCW cartoon chronology. D-Squad ends up encountering an unexpected face--CC-5576-39, aka Gregor. Gregor eventually reappears in the Rebels cartoon-but how exactly he got there still hasnt' been quite explained as of this upload. Gregor's helmet markings an homage to the hash-marks Boston Bruins goaltender Gerry Cheevers stenciled on his mask. Likewise, Gregor references the Republic Commando game series (via his armor POV helmet, etc).