3. Haunted (3.5)
Speaking of episodes that tackle Robin's rivalry with Slade, season three's Haunted is by far the one that cuts the deepest, as it makes it very clear just how toxic and detrimental to his mental health Robin's obsession with his nemesis is.
Despite the fact that Slade was (temporarily) dead and gone at this point, Robin could still not let him go and his damaged psyche convinces him that Slade has returned and is planning to destroy the city yet again. The rest of the Titans look on in sheer horror and confusion as their leader is quite literally talking to and chasing after someone who simply is not there.
This episode drives home the point that sometimes, the mental effects of a traumatic experience last much longer than the experience itself. Also, there was a lot of emphasis put on the inherent "darkness" that goes along with such inner turmoil, as Slade only appears to Robin when the lights go out.
Haunted features what is arguably Ron Pearlman's single best performance as Slade in the entire series, which is certainly saying something. Especially in episode's climax, which features a monologue by Slade's "ghost" that's enough to send chills down your spine.