Monotheism is better than polytheism because...
...it's easier. With monotheism, you never have to waste time wondering “Which god?” You'll already know.
...it's cheaper. Gods are like children: the more you have, the more it costs to keep them all happy. Think of what you could do with all that extra time and money.
...it's more unifying. Having different gods is just one more thing for people to fight about. Imagine how much more peaceful and conflict-free the world will be when everyone, everywhere, worships the same god!
...it's simpler. Think how much simpler life will be when humans are the only beings that have rights. In polytheism, you always have to be worrying about the rights of animals, trees, stones, rivers, mountains. In a monotheist world, you can forget about all that. They don't have any.
...it's more psychologically healthy. In a polytheist world, you always have to be thinking about relationships. In monotheism, on the other hand, you only have to look out for Number One.
...it better reflects existence. Look at the world around you. What do you see, Many or One?
...it's more peaceful. Look at history. Who are more likely to go to war because their gods tell them to, monotheists or polytheists?
...it's more progressive. Who created democracy, polytheists or monotheists? Civilization? Agriculture? Science? Ethics? Philosophy? Literature? Music? Medicine?
...it's morally superior. Look at the people around you. Wouldn't you say that, as a rule, monotheists are more loving, thoughtful, caring, tolerant, hospitable, respectful, and helpful than non-monotheists?
...it's stronger and more flexible. Which is easier to break, one thick stick by itself, or many thin ones together?
…it's older. Which came first, monotheism or polytheism? Which one has been around for, oh, say 150,000 years or so, which for maybe 4000?
...it's edgier. Historically, polytheisms arise out of humanity's experience of the world. Monotheisms arise out of reaction against polytheisms. Think about it: which system comes with an enemy automatically built in?
...it's unique. The unity of monotheism proves its truth.
Choose monotheism!
(Or else.)