The "Sunni" system does list rules for one to become leader, these rules include justice, piety and knowledge. However, sometimes a tyrant will assume authority by force, then you'll have to deal with the reality of the situation in the least costly way.
Most leaders in ancient as well as modern history have not been satisfactory in terms of strict religious criteria, this applies to all nations throughout the ages. This is the nature of humanity for the most part, greedy filthy men always seek positions of power. Heck, we've seen prophetic narrations discouraging us from selecting leaders who actively seek authority. The test of a tyrant oppressor proves to be a great test for pious and patient believers time and again, maybe this is God's wisdom, that the best of humanity be tested by the filth of humanity, after-all this life is a test.
If you research in the Qur'an and Sunnah, you'll find a great deal of divine advice as to how to deal with bad leaders. Our duties towards them and their duties towards us.
The means of selecting a leader are left to the experts to decide in Islam, a lot of Muslims adopt democracy as the best means of choosing leaders, I tend to prefer the system of the rule of elites. In the end, no matter what system you choose it matters not, for if the wrong person is chosen he will not govern well nor will you be pleased, if the right person is chosen he will win the people's hearts even if he is a monarch. Islam simply teaches us how to deal with certain realities in life, anything else is unrealistic and from there some people criticized the Imami belief in infallible divine heads of state.
I do stress this important point that was brought up previously by some brothers (in an unfitting way), a bad leader or corrupt one is still much better than a non-existent or absent leader. Ponder on this point objectively.