Europe as of late has turned into an authoritarian shithole and I wouldn't hold them up as a model of democracy, but I'll try to answer the question at large.
This problem was actually solved more than 2,100 years ago by a political theorist named Polybius. His answer is that no one political system is ultimately defensible, since all eventually degenerate into hideous caricatures of themselves.
Monarchy/Tyranny
If you're lucky, monarchy/autocracy/dictatorship will start out with a "good" king who's reasonable and isn't bloodthirsty. But sooner or later you'll get a tyrant. This is for a number of reasons. If it's a literal monarchy/hereditary dictatorship and the king picks his son or another close relative to succeed him, this isn't merit-based and he'll get the job even if he's a bad person. After all, it's natural for any parent to let their child inherit their kingdom and continue the family dynasty, and they might want this badly enough that they're willing to overlook said character defects.
But let's assume there isn't a hereditary scheme. If the dictator wishes or has little choice other than to retire while still alive, then this puts him in a precarious position. To safeguard himself and his family, he will choose a loyalist who won't go after him (a big part of why Putin replaced Yeltsin). Again, this concern trumps competency or decency.
But let's assume that the dictator is planning for a competent and basically decent man to succeed him when he's dead, and he never happens to retire while alive. This runs into another problem: dictators, even monarchs, frequently get overthrown and replaced by another dictator. Sooner or later, then, a person who lacks decency or competency will take the throne.
Now you might be thinking, what's one bad dictator every now and then? Well, all of the great genocides of the 20th century were done by dictators. Mao's famines, caused by his mismanagement of the heavily centralized economy (dictators left or right wing love command economies because it gives them more control and marginalizes the power of outside groups, but this also makes economic downturns more disastrous) killed up to 55 million people, the North Korean famine of the '90s killed up to 3.5 million people, Stalin's Holodomor (admittedly more intentional than the former two) killed up to 5 million Ukrainians, and the Ethiopian famine of the '80s, exacerbated by a communist dictatorship, killed more than 1 million people.
Aristocracy/Oligarchy
What about aristocracy? Well, for one thing you get some pretty massive and inescapable conflicts of interest. Traditionally, the aristocrats were just the rich, especially the rent-seeking rich (e.g. plantation bosses). So what'll they do? Enrich themselves maximally at the expense of everyone else. Perhaps bring back slavery or serfdom. Stymie needed economic reforms that come at their expense; for example, in the early to mid 19th century Russia's aristocrats held back the Industrial Revolution by not allowing peasants to leave the farms to go work in the factories. That's an outdated example, but 21st century equivalents exist, e.g. zoning laws. Again, maybe the first generation of aristocrats will be noble and not too greedy, but eventually that will change.
But suppose that, instead, you have a party-based aristocracy like in communist countries. While I'm sure there's still no small degree of nepotism, the initial selection process for who joins the aristocracy may be a lot more meritocratic. Well, for one thing, this needs some kind of ideology to justify it. If communism, they'll hold back the economy and rule with a bloody iron fist. If Chinese communism, then admittedly things are going pretty good for them right now since they're forcing capitalism at gunpoint. But China, assuming Xi Jinping doesn't make himself god-emperor for life, will eventually run into the same problem as above: either there's a massive overlap between the communist party bosses and the capitalist overlords, or you have an ascendant outside interest group which increasingly views communist rule as an obstacle to its own interests, or the fresh blood party members who aren't rich try to return things to old fashioned dysfunctional communism. One way or another, a bad scenario will happen.
But both monarchy and aristocracy have another problem: they aren't responsive to the wants and needs of the people they're in charge of governing. In a democracy, people are allowed to express their frustrations at the ballot box and through their speech, which either forces incumbent politicians to pay attention when people are unhappy or replaces them with new politicians who got elected by doing so. In short, voting and free speech aren't just about voting and free speech; these things provide useful data to the government. Undemocratic societies, which don't host free elections and where people who criticize the government risk being jailed or worse, have governments which are comparably less informed about how to serve their constituents. At best, things chug along normally but people aren't as happy as they could be. At worst, these frustrations eventually boil over into civil wars.
A good case study would be the Soviet Union. It had a large economy, but at the same time there was a perpetual shortage of all kinds of consumer goods, because the government simply didn't know what goods people wanted. Likewise, consumer goods that were available often weren't very good. Granted, this example is more an indictment of command economies than of dictatorships, but there are other examples. Europe itself could be called one; previously fringe parties like the AfD and the one in Romania grew to prominence at the polls because, due to people being afraid to break hate speech laws, the establishment didn't realize how just unpopular mass immigration from the Muslim world was, so it miscalculated and brought in more migrants in a way that ultimately backfired.
Democracy
But democracy, defined as a simple majority of the voting public being allowed to make the government do whatever they want, isn't much if any better. If you have a large middle class then democracy can perform very well, since this is truly rule by the "wisest". The middle class is mostly self-sufficient while also being egalitarian-minded and not in a position to lord over others. It has the most vested interest in economic growth and opposes both welfare robber states and stagnant plutocracies. They care about the well-being of their local communities, unlike many of the poor, while also not being rich and cosmopolitan enough to just take a plane out of the country to Switzerland or Dubai if everything collapses on itself. The poor can't afford taxes, and the rich know how to avoid them, so it's the middle class who shoulder much of the national tax burden.
But the problem with democracy is twofold: first, when the poor comprise a voting majority, they will seek to enrich themselves by using the government to steal from anyone who has more. Perhaps they'll employ self-deception so that they don't consciously realize how shitty and selfish their actions are, but either way if no one has the power to tell them "no" everything will crash and burn after a while. In this environment also, language of class conflict will take hold and social cohesion will unravel, meaning the rich and middle class (what's left of it) will stop thinking about how to positively contribute to society and focus instead on protecting themselves. Second, even where there's a large middle class, many of them will sympathize with the plight of the poor enough to want to help without being noble enough to donate their own fortunes to charity. They'll form a coalition with the poor and institute the welfare state.
What To Do
So every kind of government is self-defeating. What now? Luckily, Polybius has a prescription: the mixed government, where the strengths of one approach cancel out the fatal flaws of another. That's most Western governments today, but especially the United States. The most successful mixed governments have a strong share of democratic elements in the mixture, which is why they're colloquially dubbed "democracies" without actually being so.
And in that sense, democracies have a good track record. Here's the Human Development Index; you can check it out for yourself, and with the exceptions of Dubai (a lot of oil) and Hong Kong there aren't a lot of undemocratic states on top of the list.
Granted, this is a flawed list as it doesn't deduct points from Europe or Canada for its lack of free speech, but these countries are materially well off, at the very least, in the way that dictatorships are not.
