In my opinion the strongest anime character is Naruto; and he is about to get even stronger if you have been watching the subbed version of Shippuden. The things which I think make Naruto the strongest is the insane amount of beating he takes and he is still able to continue fighting, he does not give up and continues to fight for what he believes in. Also some of the things Naruto does while fighting an opponent is just sheer brilliance; his unpredictable nature makes him a deadly adversary. It would be difficult to compare Naruto to characters outside of the Naruto universe since each anime has things that are unique to the specific world created.
Not that Naruto is weak or anything, but do you really think he compares to a mech millions of lightyears in size that can withstand attacks as powerful as the big bang?