Although it's been frozen for quite some time now, chankast had the most impressive development this year. A DC emulator developed in 4 months that already plays quite nicely lots of games deserves the award, IMHO.
Mine goes for gcube. It came out of the dark and 2 week laters was already running comercial games in-game. Plus with Linux and Mac Support. And with the 0.3 release its even more amazing. I would say its tied with chankast but the Linux and Mac support make it go into first place.