Check http://yawestruck.blogspot.com, which lists the best YA by category. It also points to other good book sites, like "Teen Reads" and "Reading Rants". That said, below are my absolute favorites with summaries.
Noughts and Crosses by Malorie Blackman - In an alternate reality England, white-skinned people, called Noughts, are an underclass minority oppressed by a dark-skinned majority descended of Africans, called Crosses. Nought teenager Callum falls for Sephy, the daughter of a politically powerful Cross. As their romance grows, so does their safety in a society that does not tolerate racial mixing. When Callum's family is implicated in a terrorist act, the lovestruck teens face difficult choices. Regardless of your heritage, this story will lend you a new perspective on race and what it means to be human. Stunning!
The Shore of Monsters and Fall of Darkness by David J. Nix - Five generations earlier, a horde of monsters nearly obliterated humanity. All males are dead or ruined by a monster plague; words like 'father' and 'romance' have lost meaning. When teenager Sky joins an expedition to the shore that falls apart, she must survive amongst the monsters that roam the ruins. She gets unexpected help from a very surprising source. Mystery, action, and romance follow!
Jellicoe Road by Melina Marchetta - Taylor finds herself the leader of her boarding school's Underground Community, whose mission is to battle their counterparts at an all-male Cadet encampment and the public high school. For two decades the three factions have waged a game of negotiation, pranks, and sometimes physical violence for control of paths, places, and facilities. One problem: Taylor might have a crush on the leader of the Cadets. The author weaves in a back story about how the whole war started, and how Taylor is unwittingly tied to it. Romance, action, mystery, and plot twists, this is just a great story.
The Graveyard Book by Neil Gaiman - A child wanders into a graveyard after his parents' murder, where a kindly family of ghosts raise him. Under protection of the graveyard and the watchful eye of Silas, who might be a member of the undead, the boy thrives. His circumstance exposes him to a world of death, ghouls, witches, and monsters, all of which he takes in stride. When his blissful existence is threatened by the return of his parents' murderers, the boy must use his unique knowledge to survive.
And Then There Were None by Agathie Christie - The best who-done-it novel ever; one of the best twists of any story yet written. Ten people on an island retreat start to die one by one, and must determine who the murderer is before they all fall victim. Christie is the master of the twist! Even if you know the twist ahead of time, it will still surprise you. This is the standard by which all other mystery novels are judged.
The Book Thief by Markus Zusak - Remarkable story as told by Death, about the young girl Liesel's life through the World War II years. Her family dead or missing, Liesel lives with foster parents in Germany. Stealing books and food provides Liesel an escape from her grim reality, but also leads her to many friendships. Death's narrative adopts a darkly humorous manner that keeps the reader spellbound. The last line of the book is fantastic, but don't peek.
Ender's Game by Orson Scott Card - Young Ender Wiggins is recruited for a special academy that trains leaders to fight the aliens that have twice attempted to attack Earth. The training consists of zero-gravity war games between units of children, with the winners advancing up the proficiency ladder. The author tells all with frank brutality. Ender proves to be a master of the training games, and quickly ascends to leadership. The final fantastic twist is jaw dropping, making an outstanding book a classic.