I especially enjoyed "Terra Firma." It felt like a really good, emotional episode. And I liked that they brought Crichton back to Earth -- before the end of the series -- only to have very different feelings about it than one would expect. Classic stuff!
Terra Firma was one of my favorite episodes of Season four, mainly for that reason. Most shows I've seen which deal with trying to get home have it happen during the final episode or not at all. Farscape, on the other hand, achieved what it originally set out to do, but also focused on the consequences of that act. It was like closing the book on the beginning of the series, and continuing onward to what the series ended up becoming, which was a larger more engrossing canvas.