If we're talking most influential, it's obviously The Lord of the Rings - nothing else comes remotely close to its cultural impact...except Harry Potter and the Philosopher's Stone, but that hasn't had anything like the same impact on the genre itself. Distant runners up would be Alice's Adventures in Wonderland, The Lion, the Witch, and the Wardrobe, The Wizard of Oz, and A Game of Thrones.
But my personal pick would be Tigana, by Guy Gavriel Kay.
Fantasy is a funny genre - even though I love playing games with a fantasy setting, I find a lot of it hard to read, and I think it is a very idiosyncratic taste. There are books being cited in this thread that I couldn't even finish (e.g. The Name of the Wind), and I bet others will say the same of Tigana, even though I love it. I can't think of another genre where you will get so many books that are loved and hated in equal measure.