"The Justice League Recombination" focuses solely on a single plot, so it doesn't contain the usual dumb subplot that has become a hallmark of the past few seasons. To make things even better, the plot, a big hubbub over a costume contest, is hilarious. I probably laughed out loud 10 times at all the jokes from the twist of the knocking on Penny's door, the Justice League backing away, Sheldon as Flash, Raj as Aquaman, and everything in between. I don't remember the last time I enjoyed an episode of The Big Bang Theory this much.
Not only that, there is a small portion of long-term development towards Leonard and Penny getting back together, which means the writers are building on something. It's clear that Penny isn't over yet and that Zack really isn't a roadblock.