I actually know the books by the numbers better than by the names, so thank you, Phoenix for adding those in. ^_^.
As for my favourite, for some reason I have to go with #17. I don't know why that one appealed to me, but somehow it did. Maybe I just really liked the cover morph? I find it had the best intro, too. It's a good book to read when you're new to the series, which I was at the time that I first read it. (It was only the 3rd or 4th book I read)
The oatmeal plot was rediculous, of course, but it was early enough in the series that they could get away with silly plots like that.