In no particular order:
John Moore does some fun fantasy, mostly riffs on tropes. The Unhandsome Prince, Heroics for Beginners, and others I haven't read yet. They're not YA, but they read fast, and tend to be stand-alones.
Jim C. Hines wrote the Jig the Dragonslayer series (first book: Goblin Quest), a send-up of fantasy/RPG cliches starring a cowardly goblin runt. Very fun.
For YA, Eoin Colfer's Artemis Fowl books (the first three, at least) are great; a boy criminal mastermind takes on the hidden Fairy nation, which uses not only magic but technology centuries ahead of our own.
Also in YA, Jonathan Stroud's Bartimaeus books (first book: Amulet of Samarkand) are spectacular. An alt-modern word, where magicians rule the magicless masses with an iron fist... relying on enslaved spirits for their powers. Great stories, great characters, and some hilarious writing to boot, plus a pretty powerful ending to the original trilogy.
Brandon Sanderson has a YA series out, starting with Alcatraz Versus the Evil Librarians. On the surface, it's pretty silly, but there's some deeper stuff underneath it all. Plus, it's just a kick and a half to read.
Terry Pratchett's almost always a good bet for humor wrapped around interesting stories.
Douglas Adams can be light on plot, but his writing's very fun to read.
John Scalzi's pretty good at balancing wit and story, too, as is Kage Baker.
If you haven't seen Galaxy Quest, do so. And read the book by Terry Bissom; it has scenes that were filmed and cut, that add to the whole experience. Great fun, but with a good heart.
Robert Asprin's Myth series started fun, IIRC, but the later ones kinda fell apart.
Storm Front is the first.
