I've always liked the simplicity of "Let that be your last battlefield", the one with the aliens with different coloured faces. Can't honestly remember if the episode's any good, but the idea is brilliant. And it has the original Riddler.
Lots of episodes have great ideas that weren't pulled off that well. Another episode idea that's good (and also happens to be the next ep after LTBYLB) is where there's a planet suffering from overpopulation and you see the people through a window struggling for space. Cool visual, but unfortunately didn't make much sense.