Argonians certainly aren't the weakest. The Argonians were the only group to successfully push back the Daedric incursions during the Oblivion Crisis, outside of the CoC/Martin Septim. As the story goes, they charged into Oblivion with such force and might that the Daedra actually closed their gates to prevent them from being overrun by the Argonians.
Everyone else was pretty much getting owned by the Daedra until Martin closed the gates. Skyrim, Morrowind, and Summerset Isles in particular were ravaged. I can't put the Argonians at the bottom in good faith. Even though they don't have the heroes (that we know of, at least) the Dunmer or Nords had, the typical Argonian is easily a match for any other race. They also survived centuries to neglect from the Empire and subjugation by the Dunmer and came out stronger then ever.
When they attacked Oblivion, it was an unexpected, insanely large blitzkrieg-esque assault that was so big and unexpected, that it forced the Daedra to close the gates. It was a very risky move by the Hist but it paid off in the end. The Argonions had no say in the attack, they were being controlled by their tree overlords.
A typical Argonion is a joke for most races. They are skilled in very specific terrain, when not in an area that fits the bill, they'll lose to any other race.