I don't think it's completely fair to paint all dictators with the same brush. However, in general, most dictators are power-hungry and abusive. i don't think you are going to find anyone that will argue that Hitler was a positive force for the universe. As the saying goes, absolute power corrupts absolutely.
It's hard to say Hitler's army was "undefeatable" isn't it?
This depends on what you mean by a dictator. Does Lee Kuan Yew of Singapore count? In a lot of ways, he would count as a dictator to me and yet he ruled relatively well, creating a country that is prosperous and somewhat free. Plato argued that a "philosopher king" or enlightened dictator would be the best possible leader for a society. The problem is that most dictators prove the saying that "power corrupts and absolute power corrupts absolutely."
Much depends on your perspective and ultimate goals. At times you might need a strong leader to solves social issues. The Romans knew this and so they had a position called dictator under special circumstances. The ancient Greeks had this concept as well. As for Hitler, there is general consensus that he was evil. As for Mae Zedong, the jury is mixed. Some see him a a great hero, who allowed China to enter in the modern world, but other see him as oppressive. For instance, the cultural revolution was a scary time for many people.
Making the trains run on time is not much of a plus if the downside is genocide, total war, complete self-destruction and a national emotional scar deeper than the Grand Canyon.
Dictators are effective because they can sweep aside opposition and force their will on the administrative process. In the short term, it's easy for a dictator to get things done because no-one can stop them. But at what cost in the long term? They force their personal preferences on everyone and it almost always results in rebellion and bloodshed and disaster.
However, if we are looking for a good dictator, then Lee Kuan Yew, dictator of Singapore for 30 is generally held up as an example of a benign dictator, but he's a very very rare creature. Most dictators are brutal, murderous thugs. Including Hitler and Mao, obviously.
It will be wrong to say that dictators are all bad. because i believe not all dictators are bad. Look at Mao Ze tung, it was because of him china saw unity and strenghtened themselves when they most needed it. and as for Adolf hitler is concerened you must be aware that his army was known for the undefeatable unity. the good things about dictator is that he is successful in uniting people on one ground and democracy has a failing like the proverb says 'to many cooks spoil the broth'. both the extremes have good things and bad things you shouldnt just tag along with one extreme.