Basic Trope: Parents dislike one child more than his/her siblings.
Straight: Alice and Bob treat one son, Charlie, with more favor than their other son, David.
Exaggerated: Charlie is treated like a prince, with every one of his wishes granted. David has to live outside in a tent, only allowed in to clean up.
Downplayed: Alice and Bob love Charlie and David equally... it's just that, being human, they can't help but unconsciously give more praise and affection to Charlie.
Despite a secure and loving upbringing, David despises his parents and doesn't hesitate to let them know it. He humiliates them in public, does everything he can to get them in trouble, and has been known to physically attack them. Charlie is sweet natured and loyal, and regularly comes to his parents' rescue.
Alternatively, Charlie is a high-strung musical prodigy, while David is just kind of average and seems emotionally sturdy enough that his parents figure he'll understand the situation and turn out fine with minimal support.
Or in reverse, Charlie has a learning disbility that badly needs the parents' care and support. David is average, and seems emotionally sturdy enough that his parents figure he'll understand the situation and turn out fine with minimal support.
David is actually a Child by Rape. To clarify, this justifies why he is the unfavorite. It does not "justify" treating him as such for something he didn't do.
It was decided by Charlie and David's entire extended family even before either was born that the former would be the favored one.
"My brother Charles was always a sickly child. Thus, our parents always lavished attention on him, while leaving me to fend for myself and do most of the chores. When I was younger, I rather resented this. However, something about him, mostly his grey skin, thin lips and bruised eyes, inspired in me the same feeling of sympathy they so obviously felt. Looking back, I feel rather sorry for him."
Charlie and David treat their mom Alice with more favor then their dad Bob.
Carles and David show much more affection towards each other than their parents.
Subverted: Neither parent seems to have a favorite. It turns out that David is just a whiner.
Double Subverted: ...Until little Emily is born, in which case she gets the lion's share of attention.
Prior to the birth of David, his mother is seen reading a book: "Choosing a Favorite Child: Natural Selection in Action."
David is a Nobel Prize-winning millionaire astrophysicist who cured cancer. However, Alice and Bob refuse to stop talking about their little Charlie, who just got named "Unworthiest Person in the World" by CNN.
See "Justified" 6.
The parents' favorite fluctuates between/among their kids.
Neither parent seems to have a favorite. It turns out that David is just a whiner....until little Emily is born, in which case she gets the lion's share of attention. Both Charlie and David gang up on Emily. Emily's first words turn out to be "No." and she becomes Commander Contrarian. Once Alice and Bob hear enough, they decide that Emily doesn't need all the attention and decides to give the attention to Charlie and David.
Averted: They don't seem to have a favorite - and neither kid accuses them of playing favorites.
Enforced: "This family drama needs some tear jerker material. Let's make it one of those stories where the parents play favorites!"
Lampshaded: "How come Charlie never has to take out the trash?"
Invoked: "Huh, Charlie makes much better grades than David. Let's give him more attention in the hopes that it will pay off."
Exploited: David believes himself to be the Unfavorite. His manipulative girlfriend Eve uses this fear to make him believe that only she will love him, thus making him more pliable.
Defied: "We have to love our children equally, no matter what."
Discussed: "Well, given that our parents would cheerfully have killed us both if it would save our stupid brother, I think I've turned out as well as could be expected."
Conversed: "Is there anyone on this show whose parents treated their kids equally?"
Being the less preferred child takes its emotional toll on David, as he grows up unable to attach to anyone because his parents didn't love him. Meanwhile, Charlie is no better off, because the frequent attention of his parents means he's unable to grow as a person.
David develops extremely low self-esteem and latches onto anyone who will show him affection... like Eve, in the Exploited entry.
David disowns his family and leaves, never to be seen by them again.
Parental Favoritism and the resulting Sibling Rivalry have messed the teenaged David up so badly that he ends up taking his own life. The furious police officer who deals with the incident viciously berates his mother and father for their lousy parenting - but they don't care, since their favourite, Charlie, is still with them.
That's assuming that he ends his own life instead of addressing the most direct cause of his sorrows by becoming a Self-Made Orphan. He may even walk for that if he can convince the jury that his parents were abusive or neglectful. Or that he doesn't take out the only possible innocent in the equation, Charlie, in which case his parents would push for the death penalty.
Charlie and David are kidnapped. Their parents are given a Sadistic Choice. They choose Charlie without missing a beat. Charlie is brutally murdered, because the villain figures it's more dramatic that David knows his parents don't give anything even vaguely resembling a shit about him, and Charlie is dumped in the river without even a proper funeral.
Alice and Bob constantly play down Charlie's bad traits and play up his good traits, while doing the inverse with David. Eventually, the villain kidnaps Alice, Bob, and Charlie.
He gives David a choice: Either the parents or the brother dies. David unsurprisingly chooses the brother. Alice and Bob disown and abandon David for leaving their precious to his doom, rather than thanking him for saving them.
After David snaps and disowns his entire family, Charlie begins to feel that all the love his parents give him is meaningless if his brother resents him. He begins to hate his parents as much as David does for driving a wedge between them, and, due to David's assertions that they're dead to him, considers them to have "killed my brother."
David begins to consider Charlie a "parasite," and takes no pains to hide the fact. Charlie would prefer to die than put up with his brother's contempt, not to mention the pain of his condition. Alice and Bob are oblivious to either's wishes. Eventually, David demands either Charlie or their parents pay him back in some fashion, or he's happy to let Charlie rot...
Go back and clean up The Unfavourite. Then get your brother a drink. And if you piss in it again, your life won't be worth a lead penny.