I'd say the best mission is the one that gets there safe and sound and starts sending back data and images. If they all make it, then they're all the best. :-D

But for my choice, I'll have to root for Nozomi. I know I'm biased here, but for a country without a strong track record of space missions to send up such an ambitious mission really impresses me. I really hope it achieves it's objectives. I've always been partial to the underdog anyway. :-)

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere