Mine is tied between two by the same author...
The Woman That Walked Into Doors and Paddy Clarke Ha Ha Ha, both by Roddy Doyle.
They deal with some dark subject matter like bad parenting and loneliness, as well as bullying... but they handle it so well and maturely. It's all written by an irish author though so it might be a little harder to read if you're not native to ireland or the UK.
TWTWID is about the life of a girl from school to adulthood, and her journey with an abusive relationship. It's honestly beautiful and it's hard to believe it was written by a male author. A lot of my female friends relate to her struggles and she has this incredibly believable character voice. Many male authors don't do as good a job at portraying a female protagonist but this character is really engaging and interesting.
Paddy Clarke is about a young boy living in a family that's falling apart, and it portrays children in this weird but accurate way that I like. These kids are rebellious and scared of the world. They break the rules and are sometimes unkind to each other and the perspective of the young protagonist feels very raw and real. It's clearly written for an adult to read, but it really puts you in the position of a child in a situation where he is powerless.
Weirdly enough Roddy Doyle also writes children's books that are really entertaining - the man's a good writer.