What a tough choice. But if I had to, I'd probably pick Galaxy. I'll probably enjoy MP3 just as much, and Brawl will get the most playtime for sure, but Galaxy will be a very unique and inventive experience, which goes a long way in my book. I think RE:UC will be polished up into a fine game(I'm a big RE fan), and Z&W seems to be a masterpiece in the making. I'll probably end up having all 5 of them before the end of the year anyways. But just to be clear, my vote goes to Galaxy.
"I feel like I could take on the whole Empire myself."