There are many different character traits and personalities, and these play a role in which characters a reader or writer consider their favorites, or their least favorites.
For example, there are some people who like characters that are good-hearted, brave, kind, etc. On the flip side, some people get annoyed if a character has few, if any, flaws, and say that the character is too perfect or goody-goody.
A stubborn character who isn't afraid to speak their mind is also loved. Other people may find them to be rude or irritating.
And there are also people who find a strange attraction to antagonists. Whether they have a sharp tongue, are cunning, or are just cool-looking, some people enjoy these characters. Of course, there are others who just see an evil megalomaniac who is merciless, and they cheer when the bad guy is gone.
To further accentuate my point, let's take Jayfeather, a character from the 'Warriors' series by Erin Hunter. Jayfeather can be stubborn and sarcastic. He hates being treated helpless or inferior. But his wit, and his refusal to let things stop or slow him down, make him a character many people like. And despite his often prickly exterior, he cares about others and is a loyal friend. His intelligence, bravery, and good heart make him a valuable addition to the story.
In your opinion, what makes a good character, or a bad one? What makes them a favorite, or one you can't stand? Obviously character development, making them unique and realistic, is important. But to a certain extent, what makes a character good or bad is defined by the opinions of others. A character that one reader loves may be viewed as whiny and annoying by another reader. It's a unique blend of the writer's skill and effort, and the minds of the people reading the story.