Let's define "atheism" first, which luckily can be done with very few words. Atheism, in the common usage of the word, simply means the lack of belief in God. Nothing more and nothing less. Further elaboration only paraphrases the above definition or deliberates either the reasons for or the consequences of it.
From an academic point of view, a distinction can be made between people who deny the existence of God (strong atheists) and those who simply don't believe in it while accepting that it's unprovable either way (agnostics). The first doesn't make much sense, as you can't disprove God the same way as you can't disprove unicorns. So in the following, I will use the word "atheist" in the sense as defined in the previous paragraph.
And now, let's get to those arguments!
Atheism is just another religion
"You make it sound as if it was a bad thing", one might respond. But the correct response is that it's just not. It's simply the lack of belief in gods. It doesn't tell what a person thinks about politics, art, economy, psychology, or anything but religion. "How does an atheist think about social justice?" is exactly as meaningless a question as asking what a non-firefighter thinks about social justice (as opposed to a firefighter). The word "atheist" in modern times should be considered as strange as "non-monarchist". It is a negation of something fewer and fewer people are today.
As Richard Dawkins said: “We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further.”
This is not to say there are no people who put their atheism is at the center of their identities, for example, those who broke free from a very religious upbringing, but the number of such persons is insignificant compared to the masses of casual non-believers.
Atheists can't explain why the Universe exists
Absolutely true. But there is a great T-shirt inscription saying: "I Don't Know and You Don't Either". We don't know why we are here. Billions of people simply pretend they do, and this statement everyone who spends a moment to think about it must find true. Why? Because if you are a Christian, you "know" that all those Muslims, Hindus, Buddhists, etc. are wrong. If you are a Hindu, then...just replace a word.
Atheism is irrational because you can't prove there is no God
This is wrong in several ways. First, whoever says this, misunderstands who is expected to come up with some evidence in an argument. If I tell you I keep a gorilla at home (a schoolmate in first grade tried to sell this) and I want you to believe this, the burden of proof is on me. If I say nothing can travel faster than light and you disagree, the burden of proof is on you. Why, what is the difference between the two cases? The first example was about something very unlikely and therefore almost unheard of. Everyone can see what's wrong with it using common sense. Keeping a gorilla in a flat would be extremely inconvenient and probably against some laws, and would be newsworthy enough that you'd have heard of it already.
However, in the second example, I state a well-known scientific fact that has the backing of virtually all physicists in the world. Scientific theories get overturned sometimes, but that requires very strong arguments. Therefore, if you disagree with the commonly accepted limit of speed in our universe, it's you who has to come up with very strong evidence to the contrary.
Regarding the original statement, neither common sense nor evidence indicates that the world is created by any particular God. Few planetary scientists, physicists, or cosmologists are religious (and if they are, only in a liberal way) and everything we know about the origin of life flatly contradicts the Bible and all the other genesis myths.
The other problem with the statement is its assumption that the only alternative to atheism is my own God. Following this logic, since you can't prove that Odin doesn't exist, it's irrational to not believe in him.
Atheist are amoral
This asinine claim enjoys surprisingly wide support among simple-minded religious advocates and, even more surprisingly, in some vague form from Jordan Peterson (proving that a person can be brilliant in one field and very confused in another). According to the argument, the only thing keeping people from thieving, murdering, and raping is the fear of God. The statement tells more about the one who says it than about atheists.
A more philosophical version of this argument is that without God our moral values have no foundation. There is no objective right or wrong, and therefore the atheists live in the swamp of moral relativism, where the wrongness of even mass murder is only a matter of opinion. This is a question for philosophers, and whatever they have to say on the matter has little influence on how real people live their lives. Instead of speculation, we can just look at the empirical facts about whether the absence of faith make one depraved or not. They are plenty, and not supportive of the motion.
Children who have no concept of God don't go around bashing other kids on the head or stealing stuff. They do steal and hit sometimes, but they instinctively understand the concept of fairness, and quite often are nice and helpful with stranger kids. This applies to our closest relatives, the primates, and to other more intelligent species of the animal kingdom. There are both viciousness and kindness in nature, and the behavior of all sentient creatures seem to be tailored to, to varying degrees, reciprocity.
The most religious countries in the world are predominantly African, South-American, and Muslim, with crime rates through the roof or with outright state oppression. I don't think religion leads to depravity, it may well be that the miseries of life there or the iron fist of the state make people religious, but the numbers don't show that it makes people better either. If there is a genuine correlation between religiosity and crime, it's a positive one.
Or, someone who is reluctant to read up on history can just take a look at the Scandinavian countries of the present. They belong to the justest, most egalitarian societies of the world and the safest places to live. Also, they are the most atheist ones. According to surveys, only 5% of Norway's population think religion is an important part of life.
There have been many societies in history where the state religion didn't promise rewards in the afterlife (ironically one of them is the Jewish that gave birth to all monotheistic world religions) and they were not made up of rampaging lunatics. There has been no society on Earth where senseless murder, thieving, lying, or raping were condoned. There seem to be universal moral laws that are wired into us, with or without the belief in supernatural sticks and carrots.
Atheism leads to the disintegration of society
This is the only popular claim that I think is defensible. Societies don't run on laws and policing only. They need forces of cohesion to keep them together (what the former states of the Soviet Union or Yugoslavia lacked), and one of those can be religion.
But there is no evidence that religion is indispensable, especially if we look at modern European states.
Atheism led to Nazism and Communism
It's a very old, very tired, and very annoying argument, but it can give one a pause when first encountered. (It also contradicts the previous anti-atheism point - does atheism leads to tyrannies or total dissolution? Please, make up your mind)
The Nazis weren't antagonistic to Christianity, their 1920 manifesto even paid some lip service to it. Apart from the Jews, no one was persecuted for his belief in the Third Reich. Instead of rejecting faith wholesale, they channeled their society's spiritual energies into some pagan-flavored race-worship and the personal cult of the Fuhrer.
Communism, on the other hand, is an outspokenly anti-religious ideology. But there is no evidence that the terror regimes it bore had been any less monstrous if it had been neutral on the issue. And in fact, once one starts to think about it, Communism seems more and more like a religion (especially Catholicism) in surprisingly numerous aspects. It had its prophets and messiahs (Marx, Lenin), holy scriptures and dogmas (the writing of these men), heretics (Trotsky), popes blessed with infallibility (Stalin and Mao), sectarian wars (Tito vs Stalin, China vs USSR), martyrs (thousands), inquisition (show trials and purges), the ritual of confession (communist self-criticism), and finally, its followers believed to be part of something infinitely bigger and powerful than themselves.
But we don't need to speculate because history actually tells us whether atheism leads to murderous tyrannies or not. It does the opposite, as discussed in the previous point. Modern Scandinavia probably has a much smaller proportion of believers than Nazi Germany or even the Soviet Union, where despite the aggressively antagonistic state around a third of its citizens claimed to be religious.
And here we reached the end of the list. I think it covers it all. Therefore I was briefly considering removing the "stupidest" adjective from the title because these seem to be not just the worst ones but the full list of anti-atheist arguments. But they are still stupid, so I just left it like that.
