Ah now, one of my personal favourites.
In Susan Cooper's magnificent sequence of novels, The Dark is Rising, two books stand out for me. The Dark is Rising itself (number 2 of 5), and The Grey King (number 4). I have always struggled to choose which of them is my favourite, but on balance I would turn to The Dark is Rising. I find its snowy Christmas setting hard to beat.
The Grey King is, without doubt, the most beautiful of the series, with one of my favourite closing paragraphs in all literature; the story pulls at my heartstrings every time I re-read it. But the impact of Will's first adventure just wins it for me.
However, on the subject of book covers (all five were illustrated by Michael Heslop), I find The Grey King the most evocative. It's full of personal nostalgia, reminding me that this was the book (purchased from a school library clearance) that first introduced me to Will Stanton, and my joy at discovering it was just one of five stories in the sequence.
Pure magic.