It's been said many times and many ways (no, not Merry Christmas, it was just Easter yesterday, for Pete's sake!) but it's true: Characters make the story. And like everyday life and the world, it takes all kinds of characters to make a story.
Not all characters are likable. Some are fearful, annoying, and downright despised. And there are different degrees of “likableness”. Some characters are the kind most people fall in love with, some make us laugh, and others are like the friendly cashier at the supermarket—you know their names (if you pay attention to the name tags) and they're a pleasure to do business with, but once you've paid and gathered your groceries, you don't give them another thought.
So many characters, so many story possibilities....
The key to good writing is to populate your story with a mix of different kinds of characters. A novel that has nothing but wonderful, agreeable characters would become boring very quickly. Hard as it may be to believe, so would a story with nothing but characters who are unlikable. Just as in the real world and in our daily contact with other human beings (yes, writers DO get out the house on occasion), a story should have a variety of humans with a characteristics unique to each one. This doesn't mean a writer has to really stretch to come up with characters who are so unique that they ultimately make the Land of Oz seem as bland as Stepford, Connecticut (although a glance around the dinner table at Easter dinner might reinforce the idea that truth is stranger than fiction!)
Your most likable character should, for obvious reasons, be the protagonist. And with good reason; if you're asking readers to stick around for the length of the book to see what happens to the protagonist, they would prefer it to be someone they'd like to be around. Imagine yourself taking a 300-mile journey in a small car with someone you don't like. Don't do that to your reader.
Also, the writer shouldn't make the protagonist the only likable character. For one thing, that's just not the way real life is. And you don't want your readers to end up thinking that your character really needs to get out more and find better people to associate with! A writer should be exceptionally careful when the story is told in the first person point-of-view. It's not easy to have a character talk about themselves and not come across as boring or irritating (you've gone to parties; you know who I'm talking about.) A likable protagonist should come with a heaping helping of genuine humility or self-deprecating sense of humor. And they should NOT whine!
All in all, a writer should strive to create a balance of characters who serve to move the story forward and not just fill in the background. And it's more important than most realize that the protagonist, the character around which the story revolves, be someone people care about.
Because, like it or not, that's with whom many readers associate the author.