Well,It depends.Which of the Ben 10s are you talking about?If it's the original series,then he has a chance against Danny Phantom with GhostFreak,Unless he turns WayBig (But could be exposed for position),or an alien that cannot be possed.But against Aang,Avatar State? Nah,The elemets are really strong when you think about it,as shown,WayBig is vulnerable to strong opponents (Who which
Ben. While Aang can control elements, Ben can pull all kinds of surprises. Such as firing nuclear energy as NRG, and performing lava geysers. He can also duplicate as Echo Echo, and make it difficult for Aang to keep up. Plus, he can always outsmart Aang as Grey Matter or Brainstorm, both aliens have higher IQ than Stephen Hawking.