The one that will probably last you the longest is Legend of Zelda: Twilight Princess. Super Mario Galaxy is a great game and decent length; especially if you want to get 100% completion with Mario and the unlockable character.
Loz TP was my first game and gave me a great welcoming to my Wii.
But I also have Super Mario Galaxy and also was a great experience so either of them is a great choice for a first game.
Super Luigi Galaxy, LoZ: Twilight Princess, Medal of Honor Heroes 2, Super Mario Strikers, Guitar Hero 3, Metroid Prime 3.....and the Mario Kart Wii that comes out in April or something.
Twilight Princess. It was the first game that I got with my Wii back on November 17, 2006. I had to wait in line for 2 hours in the freezing cold and I was #49 Metroid Prime 3 is a must as well. I'd recommend Battalion Wars II and MOH:H2 as well.
if its gonna be your only game for awhile i suggest mario galaxy,its fun but then after like 60 stars it starts to get boring. when i get ssbb and mlb i dont know if i will still play mario galaxy as much but i will still play it. or wait abit and get mario kart