tough call...I hit my brother over the head with the ball-peen end of a 32 oz ball-peen hammer, maybe I was 4. It took both hands to raise the hammer and tomahawk him from behind. He also broke out the bottom half of my top front four teeth with the business end of a piece of timber. At one point he gave me a black eye (not uncommon), but my friend's older brother held him down while I gave him two black eyes...
but the last time we fought was when I was 17, over the remote control. I broke his nose and he popped my jaw out for second.
That doesn't count the stitches and internal bleeding associated with "playing".