In no particular order:

Forever War. This is a good companion piece and counterpoint to Starship Troopers. As a book I find it more engaging and easy to get through than Starship Troopers. There is a lot of interesting contemplation about society and government structures in the book.

Foundation. A classic novel that tackles the idea of the "great man" theory of history. The next two follow-up books round out a great trilogy. There are more books after that, but those belated sequels don't have the same spark.

Dune. It's Dune.

Hitchhiker's Guide To The Galaxy. This book and the sequels are the funniest scifi out there.

Ender's Game. A great book about a child prodigy in military strategy, and the consequences of a government which uses him.

At The Mountains Of Madness. Humanity is either a joke or a mistake.

Tactics Of Mistake. Decently breezy military scifi. It takes The Art Of War and turns many of its principles into a scifi plot.

Hyperion. Strange, scary, weird.

Old Man's War. Another military scifi that is accessible and easy to get through. Great central concept.

The Moon Is A Harsh Mistress. Robert Heinlein's best book. An uprising, but on the moon.