It's easy to imagine a dragonair laying an egg. It would be just like a snake doing so. :P
Anyways, I'm in agreement with what's already been said here - that the mammals would have live birth, reptiles would lay eggs, etc. ....with a few exceptions, anyways. A few mammals (the monotremes) do lay eggs, and some reptiles and fish (such as some chameleons and sharks) do have their young come out live - they have eggs, but the eggs hatch inside the mother's body.
Actually...with some of the bigger mammalian pokemon, such as the already-mentioned miltank...it makes no sense for them to lay eggs, unless they have really, really small babies. The eggs would be too big for their safety otherwise. O_o