Ok this thread is dedicated to Marvel's top voyeurs... The Watchers. Keep in mind that most of the feats come from Aaron the Rogue Watcher and Uatu, and both of them are pretty young by the Watcher's standards(meaning that most of the race is actually more powerful). All right here we go...
I'm sure that everyone's aware of Exitar beating the One(the head Watcher), and I know that people say that he was able to do it with ease, but I can assure you that they're wrong. Exitar actually spent several THOUSAND years prepping and gathering energies JUST to take down the lead Watcher... http://img529.imageshack.us/img529/1732/f440006gg6.jpg
"The bodies of Watchers are containers for a kind of Cosmic Primal Force,
effectively Omnipotent, Indestructible, and Immortal unless they choose to die"
Uatu had the power to stop (or at least impede) the Magus' plan of merging the universe he created with 5 Cosmic Cubes with the 616 universe, but he doesn't because of his vow of non-interference:
The 'Rogue Watcher' moves and rearranges a hundred million Stars/Planets into a net-like pattern -- even Galactus is astounded by this display of power: