I don't have a Wii so I can't play Mario Galaxy but I have played just about all the big games on the 360 and PS3 and even with all of it's problems, Mass Effect is GOTY for me. Sure there's issues with the inventory, the sidequests are all pretty similar, the dialogue wheel isn't a huge innovation, horrible elevator rides, and there's some slowdown (particularly on planets with a snowstorm) but the game manages to rise above all of these things that would kill just about any other title.
The story rocks, the graphic design is incredible, the soundtrack is mindblowingly awesome (I could listen to the galaxy map music all day), and as hard as it was initially to wrap my head around the combat when I first started, it all started to click with me as I gained more levels. I actually love the combat now.
Mass Effect >>>>>>>> KOTOR.