[Cerek]

Impossible to answer.

Professional assasins rarely reveal their real identity - even to clients. They would use codenames and middle men to maintain several degrees of seperation.

Every major city in the U.S. has a proliferation of gang members who will kill you for looking at them the wrong way (literally). That's pretty dangerous in itself, but then consider these gang members usually follow a leader that had to earn the respect (and fear) of the gang members because he is even more ruthless and deadly than they are. These guys don't have to do anytning more than point at somebody and their "homeboys" will happily blow them away.

Then there is the Mob or Mafia. Even more dangerous than gang members because these fellows have been at it a lot longer and - as their name suggests - they are "Organized" about their crime. Crossing a mafia boss for any reason is an automatic death sentence - one that is usually carried out in a very sadistic and brutal fashion. As with gang members, mafia bosses can literally have somebody killed with a snap of their fingers.

Then there are the Columbian drug lords and their private armies. These fellows make HUGE money off the drugs they sell and they protect their way of life ruthlessly.

Then, on the side of the "good guys", you have military specialists that are trained to infiltrate an enemy zone and take out a target (or targets) and get back out - all with little or no backup. They get the best and most intense training available along with access to some heavy duty artillery.

In thier own ways, every one of these men would be extremely dangerous if you were their target - so it is impossible to narrow it down to just one.