wow that's a hard question, I have a lot of favourite books... but the one that I think will always be my favourite is The Ancient Future by Traci Harding, it was the first real novel I read for pleasure, not because it was assigned in school, God that was a long time ago now, from there I was hooked and started to read books by the dozens.
My copy of The Ancient Future is all ratty and dog eared now, but it's got a pride of place on my shelf, funny thing is my family has never read it but I once heard my grandfather recommend it to one of his friends because "it got my grandson off his computer games and into reading, so it must be good".