It is a matter of taste, of course, but I recommend these three, in no particular order:
Time Enough For Love
Friday
Methuselah's Children

The novels which followed the publication of Stranger in 1961 are the closest in philosophical themes and writing style, although each one has wildly different plot and subject matter, and might be the best ones to start with.

Glory Road was Heinlein's immediate next book, and is a kind of SF take on sword and sorcery in which a soldier discovers his life has been directed towards making him into an interdimensional hero and the perfect consort for the empress of twenty universes.

Next, Podkayne Of Mars is about a teenage girl from Mars with an evil genius kid brother getting caught up in interplanetary intrigue.

A lot of mentions for The Moon Is A Harsh Mistress already; this is widely considered one of his best, and definitely has the epic sweep of Stranger and the examination of big ideas, including the nature of families, how politics and governments work, how to stage a revolution -- in this case, how the Moon becomes free of a tyrannical government on Earth.

These would be my top recommendations.

Everyone has suggested my favorites but one... "I will fear no evil."
As a side note, Johann Smith? Has anyone compiled a lineage starting with Valentine Micheal?

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