Dictatorship and a monarchy are two very different systems, mainly in the way they are enforced. A dictatorship is primarily enforced militarily (because otherwise, most all dictatorships would be overthrown - they NEED the military power), monarchies are enforced primarily by tradition and keeping the upper class happy. A monarchy is much preferable to a Dictatorship.
Oh, and I don't think you know what a constitutional monarchy is, and are confusing it with a normal monarchy. A constitutional monarchy is basically Canada right now. The Queen/King has power only in name, they have no ability to pass any laws or regulations whatsoever, this right is left within the hands of an elected parliament.
Democracy gives each and every person in a society equal say in what happens in their lives. This is a fact.
Communism takes this same freedom of choice, individuality and strength away from each and every person and puts it into the hands of a small group of individuals who head the Communist state/country. This is also a fact.
This, alone, is the reason Democracy is politically better than Communism. All outlying issues with Democracy and Communism come from these two, short simplifications. Democracy has freedom of choice, which people need in order to have satisfactory lives. Communism does not.
Economically, Communism is worse than Capitalism (which is the typical system espoused by a politically Democratic country) because it offers no opportunity or reward for advancement. We need that. As human beings, we need that as motivation to actually work and put an effort into our lives and jobs. We need a reason to continue trying.
Now, I know what you're getting at. Realistically, Democracy in action has a lot of flaws. Some rather serious ones too. One that consistently springs into my own mind is that putting power into the hands of the easily suggestible and frankly, stupid, majority of the population is pure lunacy. This is true. This is also most of the reason why politicians lie. I would lie too, if the only way I had to get any power or fulfill my life goals was to have an idiot with idiotic requests put me there.
But what you're saying - that it creates an omnipotent majority that can impose itself upon the minority - is only seasonally true. Because Democracy is a system that advocates change (primarily through the election of multiple different leaders - though having two terms was a bad idea in the US, since it encourages presidents to only actually make an effort on their second term, if they make it that far), it makes it essentially impossible for one majority to continue swaying another minority, because that majority is constantly in a state of change itself. It's why the US hasn't had only democratic or only republican presidents for the entirety of its history.
What you mention there are not examples of political oppression, and your belief in such is highly displaced. They are examples of SOCIAL oppression. Understand this: social beliefs will always transcend political systems, regardless of the society's political system.
Consider criticism of homosexuals in Russia. The majority of the population still holds to outdated beliefs that homosexuals are wrong, and should be punished for it. Do you think Putin has any influence on this? Or do you think it is a result of literally millennia of belief and maltreatment of homosexual persons?
Similarly, mistreatment of the black population in the US (and before that, the British empire) was a social belief that held through multiple monarchies, a republic, and a democratic system. They were not political. They have never been political, and they will rarely, if ever, be political. They were an example of traditionalistic views and the common human desire to create an "Us vs Them" mentality in action, and nothing more. They are, as well, entirely irrelevant to a discussion regarding the quality of political systems.
Now, as for why a certain political system is better than another, this should also depend on your beliefs of what is necessary to make a political system better than another. Is it enforcing equality? Allowing for advancement? Allowing for progress? Facility of action in times of emergency?
I think that, for the most part, Democracy allows for all of these far more than any other political system in the world - save that of the last one, where decisions are bogged down due to the pure magnitude of input. This is why, in times of emergency, it is necessary to act without the people's wants and needs in mind, and it is something that, currently, the US fails at (see Congress).
However, understand that the US is not the only example of Democracy in the world. It is only the biggest and most popular, as such it is also prone to the greater flaws and issues that will strike at any political system when it acquires more influence.
For much smaller and less size-stricken democracies you can look to the scandinavian region, where, by all accounts, other than taxes, life is idyllic.
You seem to be making decisions based on arbitrary judgments of systems you don't fully understand. You're not quite looking deeply enough into whatever system you choose to criticize, and it makes your criticism summarily shallow, weak and uninformed. I would suggest thinking a bit more about your accusations before putting them to light.
