Hmm... here's some of my favorites:
Blue Sword and Deerskin by Robin McKinley taught me its ok to start a character weak and put them through hell to make them ready for the real battle.
Chronicles of Narnia was my first love of Fantasy.
Gai-jin by James Clavell taught me that 'Yes, you can be historically accurate while following fictional and flawed characters to their doom'.
The Historian by Elizabeth Kostova took me through history and places I still want to visit and showed you can add just a little paranormal flair with huge impact.
Captive Prince Trilogy by CS Pacat gave me a coupling that knew who they were and never once apologized for it.
Black Dagger Brotherhood by JR Ward reminded me of how much I adore narrative where the character's voices and thinking disrupt or take over at any given moment for both emotional impact and humorous jabs. (WRATH FOREVER!)