When the Huygens probe parachutes through the atmosphere of Saturn's moon Titan, letting us see what things are like from the surface (or near surface, anyway). I was nine years old in 1976 when the Viking landers made it to Mars. I remember writing NASA for information and feeling my eyes bug out of my head when they sent me a bunch of 8"x11" glossy photographs of the view from the landing sites. I've been a space nut ever since - hard facts eventually overcame my bias so that I grew to oppose the money-wasting, minimal science, manned space program, but I can't get enough of robotic exploration.
Huygens is the coolest since Viking because it will give a human's, groundview, perspective of being on another world. I'm not talking about scientific value, although it will have plenty of that, but the the feeling of being there. Pictures from orbit are great, but they don't have quite that same sense of personal exploration.
The other cool thing is that Huygens will have a microphone, so we'll hear sounds from another world. The only time we sent a microphone to Mars was on the failed, badly-built Mars Polar Lander in 1999, that crashed on the surface. No sounds came back from that one. Probably the only thing that could be heard is the probe's computer as it crashed down: "AAAAAAAAAAAHHHHHHHHHHHHHHHH!!!! DAMN YOU NASA!!!!!!!!!! AAAAAAAAHHHHHHHHHHHHH-"
Let's hope for better luck this time.