Francis by far. he got socked by a tank, the same thing that tosses cars with ease and walked away with minor bruises. He also has the best use of the words "Merry Christmas" as you can see here:
YouTube video
Nick hasn't done much...
For comparison:
I remember once playing Left 4 Dead with a friend of mine, Francis was an AI.
When in a building, I saw the words "Francis has startled the Witch" appear, and thought that I was gonna have to bail him out. I went into the room they were in. The Witch was dead. In all of two seconds.
Later in the game, me and my friend were getting their asses raepd by zombies, along with Zoey who was also AI, Francis was killing them all while healing all of us, because he is that ****ing good.
True story.