Four arms is Ben's strongest alien ever
he doesn't have 1
The strongest creature is Alien X.
Alien X somepeople argue that it may be Waybig
There are 3 series : Ben 10 (original) Ben 10: Alien Force (2nd series) Ben 10: Ultimate Alien (3rd series)
he is humongousaur
Ben 10's watch is an alien device called the Omnitrix.