The very first Heinlein book I read was The Cat Who Walks Through Walls. It featured Lazarus Long, and I went on to read many of his books.

Starship Troopers is a quick read, well worth it.

Stranger in a Strange Land is an all time classic novel, a MUST read.

Time Enough for Love

Have Spacesuit will Travel

Anything really.

It’s hard not to recommend “Starship Troopers” It is a very tight little book which is foundational for a bunch of other good sci fi.

One of his short stories from '41 "By his bootstraps" is a great time travel paradox.

And The moon is a Harsh Mistress solid story and a great into to orbital mechanics...

In my opinion, his earlier stories were alot easier to read. To this day I still recommend Starship Troopers and Citizen of the Galaxy, not just to anyone new to Heinlein, but anyone looking for a good story to read.

I like his early 50s-60s work (some were written for youth):

The Star Beast

Tunnel in the Sky

Starship Troopers

The Moon Is a Harsh Mistress

And then there is the whole Lazarus Long saga, which contains some controversial sexual ideas (incest/pedo). IMO the first books in the saga are better.

Depends entirely on their age and tastes. But if I know nothing of them, Double Star. It's quality without being too pushed in any particular direction.