"Congratulations on your bundle of joy" always works. Things like "Good luck" and "Best wishes" could be taken sarcastically. It hardly is but some people can and may see it as "Good luck with that" in an uncaring or rude manner.
For a baby shower, "best wishes" is a good choice. "Congratulations" is also appropriate for a baby shower or new baby greeting card. You can also say that you are happy for the new parents.
