I made a little cheerful list of my favourite first lines, and surprisingly it turns out they are all childrens books.
THE SECRET GARDEN, FRANCES HODGSON BURNETT
"When Mary Lennox was sent to Misselthwaite Manor to live with her uncle everybody said she was the most disagreeable-looking child ever seen."
CHARLOTTE'S WEB, E.B. WHITE
"Where's Papa going with that axe?" said Fern to her mother as they were setting the table for breakfast.
THE BAD BEGINNING, LEMONY SNICKET
“If you are interested in stories with happy endings, you would be better off reading some other book.”
THE GRAVEYARD BOOK, NEIL GAIMAN
“There was a hand in the darkness, and it held a knife.”
OATH BREAKER, MICHELLE PAVER
“Sometimes there’s no warning.”
A MONSTER CALLS, PATRICK NESS, Inspired by an idea from Siobhan Dowd
“The monster showed up after midnight. As they do.”
But my all-time favourite is still the opening to,
THE PEPPERMINT PIG, NINA BAWDEN
'Old Granny Greengrass had her finger chopped off in the butcher's when she was buying half a leg of lamb. She had pointed to the place where she wanted her joint to be cut but then she decided she needed a bigger piece and pointed again. Unfortunately, Mr Grummett, the butcher, was already bringing his sharp chopper down. He chopped straight through her finger and it flew like a snapped twig into a pile of sawdust in the corner of the shop. It was hard to tell who was more surprised, Granny Greengrass or the butcher. But she didn't blame him. She said, 'I could never make up my mind and stick to it Mr Grummett, that's always been my trouble.''