The prototypical "fairy tale" castle is Neuschwanstein in Germany. It's graceful and in beautiful terrain. Its date of building (19th C.) disqualifies it in my mind from being a true castle, but it's a remarkable site nonetheless.
I've always admired the Edwardian castles--Harlech, Conwy, Caernarfon, Beaumaris--in Wales, built in the late 13th C. to secure British rule. These were designed by Master James of St. George, who popularized concentric and symmetrical designs.
Caernarfon is probably the most magnificent castle, with its banded colors of stone and angular towers said to be patterned after Constantinople. Its Eagle Tower is awe-inspiring--hexagonal construction with three small turrets.
Beaumaris is probably the most perfectly planned and executed concentric castle (though never finished). Its concentric layout, strong gates, and moat are extremely defensible, and a dock and gunner's walk allow for restocking by ocean. If I were a medieval lord looking to beat back attackers, I'd pick Beaumaris.