I'll stick with just fiction, and not count book series from TV shows, etc.
Favorites:
Alice books by Lewis Carrol
Replica series by Marilyn Kaye
Princess Diaries series by Meg Cabot
All-American Girl by Meg Cabot
Are You There God? It's Me Margaret by Judy Blume
Deenie by Judy Blume
The Sea Fairies by L. Frank Baum
Sweet Valley High series by Francine Pascal
Ella Enchanted by Gail Carson Levine
Elsie Dinsmore by Martha Finley
A Life Of Faith series
American Girls Collection, especially Kit, Felicity, Samantha, and Addy
Generation Girl series
History Mysteries series
Least Favorites:
How To Eat Fried Worms by Thomas Rockwell
Maniac McGee by Myrna Peters
The Last Battle by C.S. Lewis(sorry, all you Narnia/Lewis fans!)
Jane Eyre by Charlotte Bronte
Animal Farm by George Orwell
The Bridge To Terebithia by Katherine Paterson(sp?)
Where The Red Fern Grows by Wilson Rawls
Mommie Dearest by Christina Crawford
Left Behind series by Tim LaHaye(no offense, any LB fans!)
Aquamarine by Alice Hoffman