Maya is the sexiest—and also the first character I chose to roll as in the recently released sequel to Gearbox's hit FPS/RPG hybrid—so she's obviously my first choice in terms of wallpaper.
But the rest of these designs made by CodyAWilliams are just as suitable. Though, I personally prefer the more colorful designs.
Take a look at each in their full sizes below:
Borderlands 2 Wallpaper [deviantART via Reddit]