I wanna say #21, because I have a Pyramid Head fetish. That helmet looks uncomfortable as HELL, too. Kudos.
At 50 seconds on the video, let's play (dun dun dun dun dun):
SPOT THE ONE AWKWARD GUY OUT!
No, seriously, it's kind of adorable to see an entire team of Commander Shepards, Talis and Geth, with just one little master chief in the middle. Awww...
Don't know which one I liked more, the Hello Kitty Marcus Fenix or the Mass Effect squad complete with both a MaleShep AND a FemShep (who happened to be an infiltrator, my favorite class :3). The little kid in the Master Chief costume reminded me of the midget Crimson Lance guys from the Fort Knoxx expansion to Borderlands. :P
The Harley Quinn cosplayer was my favorite.
I'm actually thinking of cosplaying as Noob Saibot for Steel City Con in two months. Of course, I'll have to get the mask... and costume...