This is one of the most difficult questions I have ever come across. Heinlein has had my utmost respect for ages as very nearly the found of Modern Science Fiction- if you could patent ideas, 95% of SF authors there today would be paying royalties to his heirs to this day. I find it rather impossible to choose... Nearing the top of the list would be A Stranger in a Strange Land, Time Enough For Love, Have Spacesuit, Will Travel(A bias, as it was my first Heinlein novel, at the age of 5.) The Moon is a Harsh Mistress, Citizen of the Galaxy, and The Door Into Summer. For someone with a mindset like mine, one novel is probably impossible. Ah, well. I tried!