Starting off this list on a relatively lighthearted tone, Mad Mod (alternatively titled Detention) is not only an incredibly fun episode, but easily one of the single most creative that the series had to offer.
The episode takes a turn for the weird early on, as the Titans are forced to navigate the titular villain's bizarre, surrealist-inspired lair with no shortage of trippy visuals as well as plenty of references to classic pieces of art.
Mad Mod's motivations really play to the "teen" aspect of Teen Titans, as he declares the team as nothing but bratty, do-gooding kids who undermine hardworking villains such as himself. Plus, the reveal that Mod is actually much older than he appears only drives this home, making it clear that he's a bitter man who envies the Titans' youth and exuberance.
Mod himself also makes a very fun villain. Between his overplayed charisma and British twang, he is incredibly entertaining every time he's on screen. And the fact that he's voiced by Malcolm McDowell certainly doesn't hurt.
A film-loving wrestling fan from west Texas who will live and die by the statement that Return of the Jedi is the best Star Wars movie and unironically cherishes the brief moment and time when Deuce & Domino were WWE Tag Team Champions. Hates honey, but loves honey mustard.