I'd recommend The Moon is a Harsh Mistress which is a romp imho, and also happens to be a better insight into his personal politics.
Also, Have Spacesuit Will Travel, Farmer in the Sky, Stranger in a Strange Land and mostly everything by Heinlein, though there was arguably a bit of a drop in quality towards the late end of his career.
I like to think of it as 'early' Heinlein and 'late' Heinlein - although both periods are very enjoyable, sometimes it as if it's two different authors.
I'd recommend The Moon is a Harsh Mistress which is a romp imho, and also happens to be a better insight into his personal politics.
Also, Have Spacesuit Will Travel, Farmer in the Sky, Stranger in a Strange Land and mostly everything by Heinlein, though there was arguably a bit of a drop in quality towards the late end of his career.