Obsession, TOS 2
“Obsession” has the benefit of being one of the first Trek episodes I ever saw, so I therefore think very highly of it. Sure, there are some real doozy plot holes (like how does Kirk suddenly have empathic abilities to know whether gaseous clouds are intelligent or not?!?) and it is very convenient that said gaseous cloud kills everything it touches — except when it touches important characters (then it only stuns them.) Still, we see Kirk go a little bananas as he chases his white whale — I mean, cloud. Plus he has a feel good message about not blaming yourself too much for prior screw ups. What a guy!