Antagonists come in all shapes and sizes, and sometimes aren't even a thing you can see or touch.
They are something or someone that interferes with your characters ability to achieve his/her goals or desires. The antagonist can also be determined by the genre you choose to write in.
For example, Fantasy normally involves a clearly defined evil being introduced early in the story, who is plotting to destroy the world, or subjugate all races under their rule. The protagonist's role then is to eventually defeat this evil. Conversely, a well written mystery will keep both the reader and the main character guessing until the end. The purpose is to keep the antagonist hidden while providing clues in the hopes that he will be un-masked at the end.
Often in literary fiction, we find that the antagonist can be an idea or emotion. Such as the story of parents dealing with the emotion of losing a child, or widows who have banded together to deal with the loss of their husbands. Maybe a small community is divided by the idea of changing the direction of their town. One side wants change, and the other desires to keep things the way they are. Neither side is right or wrong since it's the idea that's the antagonist.
Sometimes I wonder if the popularity of romance is that the antagonist can be any or all of these things. Anything that prevents the romance from progressing can be seen as the antagonist, be it a person, an idea, or even a physical location.
As a reader, I want to see the characters grow by learning from their mistakes, fail and keep trying, and eventually reach their goal. Your characters will never achieve this growth without an antagonist to thwart their every move. Minus the opposition, a story becomes like a newspaper article, a listing of the persons involved, and the facts pertinent to what happened. A story with no beginning, and no end.
I'll be the first to admit that I lack the intelligence or deviousness to write a mystery. I have written several short literary fictions, but the emotions involved make it difficult for me. Fantasy is where I find the most pleasure and comfort because of the simple "good vs. evil" theme.
I firmly believe that if you want your story to resonate with readers, an antagonist that fits your genre is a necessity. How well you present this will make or break your tale.
As always, this is just my opinion and thanks for reading.