My first visit to this thread and it's already down to the last 4. I cannot abide The Inner Light and Darmok biting the dust before Silicon Avatar. Come on, people!
Well, of the four, I choose Conundrum. It's a strong episode, despite the fact that it exposes a vulnerability so absurd that it seems impossible for this not to happen over and over again. Some random alien from a podunk technologically inferior species can approach the flagship of the Federation and instantly wipe out everyone's memories (including Data!) and manipulate the computer to fight his war for him? Oooooooooookaaaaaay. Getting beaten up by obsolete Birds of Prey in Rascals and Generations is bad enough, but this is embarrassing.