Mmm, it's kinda tricky. I actually see four options:
A) Shep saves the galaxy, lives happily and awesome ever after...
A1) with his/her mate of choice
A2) without his/her mate of choice, because she/he/Liara died in the process
B) Shep saves the galaxy, dies (awesomely) in the process, with the surviving members of the team mourning him, and possibly, but only in the event of a male Shepard or a female in love with Liara, a possible offspring being told how awesome Shepard was.
C) Shepard fails to save the galaxy, dies in the process. Erm... I forgot, awesomely of course.
D) Shepard fails to save the galaxy, survives the process, of course eventually dies in misery when the Reapers end all life. Shepard could be stored as a construct for the next hero saviour of the galaxy, to find it and be told "Oh, I was THIS close to actually winning..."
Personally, I see A1 as kinda lame, but reasonably acceptable. After all that action this can only end with Shepard kissing the barrel of a gun and pulling the trigger, unable to find a life for him/herself in peace.
A2 I like, gives a depth of tragedy to the story, winning the galaxy but overall losing your life, which I see a fitting end to the hero who sacrifices all that's important for him/her for the greater good. If Shepard's love interest dies because of Shepard's decissions, or indeed because Shepard sent them to die, even better.
B I can't decide, could be very cool, going back to my previous lines about sacrificing all and that, or kinda lame. The part with the offspring is (I hope) a joke.
C is cool, but of course I guess it qualifies as losing the game, if indeed you can lose in an RPG, which of course I know you can't.
D... disregard