I guess a paintball to the neck just under the ear hurt the worst. But my worst injury while playing was when I tried to turn a sharp corner while running and slipped in the mud. I ended up flying and landed right on my shoulder. I saw stars and couldn't move right away.
So I"m lying there on the ground, holding my left shoulder and Tim runs over and yell something like "oh my god, is he having a heart attack??" It took me a few seconds to get it together and talk, so it was pretty stupid there for a while.