If I could only have one of those I'd go with Super Mario Galaxy. I'm not sold yet that Assasin's Creed will have GOY potential, but I think Mass Effect and Bioshock (why is this not on the list?!) both have decent shots along with Mario Galaxy. I'd say Mario if anything just because it is probably the most different of the type of games you've been playing recently and I truly feel it is going to be amazing.
You might listen to IGN's Wii Editor's thoughts here: (I haven't yet).
http://wii.ign.com/articles/832/832465p1.html
Make it easy on yourself and get Mass Effect and SMG as they are both probably sure classics and wait to see if Assasin's Creed gets amazing reviews.