Madeleine L'Engle - A Wrinkle in Time, A Wind in the Door, etc.

Ursula K. Le Guin - Earthsea trilogy

C.S. Lewis - Chronicles of Narnia

Lloyd Alexander - Prydain Chronicles

I second anything in the Wrinkle in Time series or Narnia, as Jason suggested.

The Chrestomanci series by Diana Wynne Jones

My kids all LOVE the Fablehaven series by Brandon Mull.

They all love Harry Potter as well, though some of the later books may be too dark and scary for younger kids.

We've also read the Chronicles of Prydain by Lloyd Alexander to them and they liked it.

They also like Percy Jackson and the Olympians.

Sharon Shinn has a nice series for kids.  Safe Keeper's Secret.  Also Jane Yolen. 

I know its sci fi but Hitchhikers guide to the galaxy was read to me as a kid and I loved it..

Another one that is better read aloud is Eragon, the book itself can be rather dry but if you can do character voices it really helps.

His dark materials- great series and better aloud.

I agree with everyone who recommended the Chronicles of Narnia. Also, I see that you've started The Twenty-One Balloons. That one is REALLY good--but many children are put off by the "boring patch" at the beginning. I was when I read it, but my dad told me to keep going, so I did, and I really liked it. Then my little brother  started reading it, and the same thing happened. I told HIM he needed to keep going, and he did. And he liked it. So that's my advice for that one. *Grin*

Oh--have you read any Roald Dahl? I don't know if you're looking for silly stuff like that, but Charlie and the Chocolate Factory and The Great Glass Elevator, as well as James and the Giant Peach are FANTASTIC read-alouds.

"The Door Within" Trilogy by Bastian

Series of Unfortunate Events

I also agree..Chronicles of Narnia...I would start there first as well as all the Roald Dahl books, especially the BFG if you have boys!

The Duke: "He breaks up minstrels in his soup, like crackers."

The Prince (disguised as a minstrel): "He had grown weary of rich attire and banquets and tournaments and the available princesses of his own realm.'

The Princess: "Princess Saralinda was tall, with freesias in her dark hair, and she wore serenity brightly like the rainbow."

I second all of the CS Lewis People.  Anything by Diana Wynne Jones or Loyd Alexander would be awesome.  "The Wonderful O" would be hilarious read out loud too.  William Goldman's version of "The Princess Bride" would be a fun read as well.