6. Season Four
Season four is where Clone Wars really started to lean into its dark tone, sending its characters through hell as the war around them began to escalate beyond their control. The Battle of Umbarra arc, surrounding Rex and his men's clash with the vicious Jedi Master Krell, still stands as one of the best story arcs of the show, and watching Anakin's distrust of the Council grow is also a serious highpoint.
This season also delves further into Ventress' background and brings back the character who would go on to become the show's strongest addition, Maul.
Unfortunately, for every episode of watching Rex deal with the complexities of his duty and of seeing Maul losing his mind over Obi-Wan, there are light droid-centric episodes such as "Mercy Mission" and "Nomad Droids," neither of which mesh well with the overall feel of the season. Also, it ends on something of a cliffhanger, setting Maul up as the show's new big bad but not doing enough with him to justify bringing him back.
Best Episodes: Carnage of Krell, Revenge, Massacre