Heinlein may have been somewhat weak on plots in general, but
Mistress, at least, had a very strong plot. I would say
Have Spacesuit, Will Travel was another strongly plotted book. Perhaps I just held his other works to too high a standard. His own damn fault!
Agreed that
Citizen of the Galaxy is another of Heinlein's finest, and probably the most "mature" (whatever that's supposed to mean) of the "juveniles."
Starman Jones is interesting partially
because of the analog computers, I've always thought. (But then, I taught myself to use a slide rule based partially on Heinlein's descriptions of them.) And
The Star Beast is such a funny and revealing look at politics and bureaucracy, I still refer back to it when I want to remind myself how things "really" work.
The least memorable of the YA books, to my mind, was
Time for the Stars. Not that it was a bad book, just not one of the favorites I come back to over and over.
Rocket Ship Galileo was another slightly wobbly one. Not bad, just not as good as
Farmer in the Sky or
The Rolling Stones (weak plotting in both of these, I think, but great characters).
Dennis, your suspicions about Willis being a Martian nymph are confirmed in the uncut version of
Red Planet, I think. (Though there was some very strong hinting even in the earlier release.)