"The Locket" is probably my favorite standalone episode of the series. I love Stark. I love seeing Crichton and Aeryn growing old together (although Crichton's old guy makeup is absolutely terrifying!).
The last 5 episodes of the season are absolutely fantastic and full of a lot of crazy twists and turns, and the "Liars, Guns, and Money" trilogy puts some important focus on characters other than Crichton and Aeryn. The whole story is, in fact, motivated by D'Argo. My favorite thing about Farscape is that, while Crichton is obviously the main character, it truly is an ensemble show. Every character gets their moment in the sun, and the situations are driven by their very realistic motivations.