Although, personally, I wouldn't classify the Hermes Birkin and Chanel 2.55 as 'it' bags; as they were first created well before the term 'it bag' was coined.
I also feel that the bag that caused the term to be coined, in the first place, should definitely be mentioned in a top ten of 'it bags', which I believe was the Fendi Baguette, wasn't it?
I don't think I would have included the Prada Gauffre in a top ten list, either, as I'm sure there have been far bigger selling/more in-demand bags than that (a few other Chloes, like the Bracelet Bag, Silverado and Edith spring to mind, for example. Or, as others have mentioned, the JC Ramona).
I don't seem to really do 'it' bags very much, myself, I only own one bag on this list and even that is a variation on the original (a fishscale Mini Novak with Dragon Clasp).