We have a tie! I couldn't choose between the next two, so I included them both.
Venus orbits the Sun, but not exactly on the same plane as the Earth, so it only passes directly between us and the Sun--what astronomers call a transit; think of it as a "mini-eclipse"--every century or so (and then, due to the odd dance of gravity, it happens in pairs separated by 8 years).
The last transit was in 2004, and I must admit to some bias since I saw this event myself as the Sun rose that cold morning. This image brought me right back to that moment, as I saw the black disc of our sister planet crossing the blazing disk of the rising Sun. It's a dramatic shot of what is almost literally a once-in-a-lifetime event.