Well, to me, the most ideal alien would share their knowledge to humanity and hopefully we could become a better and stronger species.
But I've thought about this, if the aliens were just another species that were going to spread their kind across the universe then I would want it to happen like on this video
http://www.youtube.com/watch?v=DU91POX33aE
At least in this form we could live on forever at least until we either sucuumb to disease or a great cataclysmic event hits Earth.