Counter point: singapore.
Oh, to clarify, I want a longer time frame. I want a autocratic regime which didn't hit the "Dumbo peak" within 1 century, +/- 20 years. Heck, even Singapore doesn't qualify, their system has been up for only about 60 years. They still have about as many years to hit a dud.
I'll wait :)
Or are you assuming that democracy naturally devolves into a corrupt regime? I doubt it. It seems to be the exact opposite: stable, long term democracies are the most resilient ones.
All governments regardless of their nature or intent devolve into corrupt regimes over time. The nature of government - authority through a monopoly on violence and coercive taxation - makes corruption, authoritarianism and collapse inevitable.
Checks and balances only work when both the people insist on them and the government recognizes them. But in a democracy, people can be convinced to vote against their own interests, to vote out checks and balances or vote in autocrats or extremists, or to simply not care to stop power concentrating or collaborating where it shouldn't.
Given a choice between statism (a monopoly on violence) and anarchy (a free market of violence), I would more trust a democratic form of a monopoly on violence, so I would agree... at least that democracy is the least worse option, depending on what you want.
But I also believe democracies tend to devolve into autocracies the larger and more complex they become and the more abstracted the machinery of power becomes from the voters, so I would add a caveat that democracies when limited in their scope and power are the best option, and that one should expect that, inevitably, the entire thing will rot on the vine and have to be done away with entirely.
Although I wouldn't go as far as Thomas Jefferson and say a democracy should have a revolution every 20 years, I think the concept of a regular (nonviolent) "reset" in order to keep government from growing too removed from the will of the people is worth considering.