If it's a choice of any animal then I agree with Otaku Gamer and it's dragons (but then you all guessed that didn't you?).
Otherwise it'll be dogs, we always had a dog in our house. Sadly I don't have one now because I now live on my own and as I'm out a lot and go to places I couldn't take a dog it wouldn't be fair or kind to leave the dog alone in the house.
I would never have a cat it's not that I dislike cats, they're quite nice animals, but they have a habit of jumping up onto things and any cat that jumped up onto my model railway would be seriously in trouble!!