As the question is asked, I have to vote for the soft-landing Mars probes, simply because the level of Rube Goldbergian engineering is so delightful and -- clearly -- executed amazingly well.
My favorite mission.... that's harder. I watched Mercury flow into Gemini and then into Apollo... and watched that drive an entire generation to greater awareness of and interest in science and engineering and related fields. We desperately need to set ourselves a challenge of that sort again.
The next Man on the Moon is going to be Chinese.