Most dangerous one I ever took part in was definitely parkour. I thought I was going to die on a few occasions.

Most injuries I've seen are from skateboarding, but it's not exactly deadly. Deadliest has to be like free rock climbing (pretty sure that's what it's called). It's not some obscure 'sport', and comes with plenty of risks - since you go without a harness or rope. Spelunking seems dangerous as well, but that's more of a hobby than a real sport. Just my two cents.