Well I realize this will make me sound extremely lame, but if I was completely honest, I would say Orlando / Disneyworld. Well maybe not in the summer months, but almost any other time I would go.
We went to DW in October and all I can say is that we got bit by the bug. The whole family had a fantastic time and we are itching to go back.
Alaska would be a close second. If traveling in the summer time, than Alaska would be our #1 pick.