I know this a really old thread but I'll give my honest opinion. Mei is weak willed but she was being realistic - even Onoki realized that their efforts were futile. And to tell you the truth, the gokage are comparable in terms of power, although Onoki is in a clas of his own.
All have their strengths. Onoki, Mei and Gaara are all very versatile. Tsunade is an excellent support ninja who can defend herself although her average speed does not allow her to take advantage of her strength. Ei - eh. Yes he's fast and strong but thats all he has. A one trick pony. In reality, he was the most useless kage against Madara - he could not do anything without the assistance of Onoki and his weight altering abilities.