I went to see The Hobbit last night (Loved it!) and seeing the eagles made me think of the Lord of the Rings plot hole (Why they did not just catch a ride on them to Mordor) but it got me and my friends thinking on what was the biggest/ worst plot hole.
For me it is Harry Potter. Voldemort spends about 15-16 years dead and DUMBledor spends the whole time NOT looking for his soul things (cannot remember the stupid name for them) and destroying them, he does not spend the time actually teaching Harry any usefull or powerfull magic, he does not even bother telling Harry about them until a year after Voldemort returns, he spends the whole time sitting on his arse doing NOTHING!
EDIT: I accept that I was wrong about the Lord of the Rings. So please do not quote me explaining why I am wrong. I have already had 10 people do so :)