Oh come on, just ONE? Here are my four tops...
"Close Your Eyes" by Kate Banks, illustrated by Georg Hallensleben, 2002, Frances Foster Books.
(Quiet, gentle, loving, lyrical, gorgeous illustrations)
"Down by the Cool of the Pool" by Tony Mitton, illustrated by Guy Parker-Rees, 2001, Orchard Books.
(Fun, bouncy, great to read aloud, kids love it)
"Pog and the Birdies" by Jane Simmons, 2005, Orchard Books.
(Great story, gentle humor, lovely illustrations and nice theme)
"Some Dogs Do" by Jez Alborough, 2003, Walker Books.
(fun, great to read aloud, I love the way that something impossible is simply portrayed as an everyday event)