Not a huge Rayquaza fan, really dislike the creation trio, so that leaves the tao trio. And Reshiram is my favourite of those 'cause it's a fire type and I think it looks better, so I guess Reshiram will be my answer.
I really like the Creation trio, out of this group, Dialga is my favorite. But the Tao trio really comes out on top for having the originality in Pokemon at the time, but out of the Tao trio, Kyurem is easily my favorite simply because he seems to have a lot more thought put into than the other members if you look deep enough. Being the body of the Original Dragon that was split apart and the fact that he was the first Pokemon to actually fuse without it being an evolution really helped in making him my favorite Pokemon of all time. In speaking of fusion, we have yet to see his Original Dragon form, Black and White Kyurem are closer to that, but not close enough. A day must come in which Kyurem fuses with both Reshiram and Zekrom to free himself from the horrible ice he was forced to endure.