I’ve read most of them, and I must say that I’m impressed by the list. A good half of the works are ones that I didn’t care about or outright didn’t like, but this is not a “best” list. As far as “essential” goes, they ought to be included.

If I may, I would only do a few changes.

To remove:

• The Empress of Salt and Fortune - This is a historical fiction with a tiny little magic gimmick thrown in, which largely serves only to gloss over the worst logical inconsistencies. That should disqualify it from the list, but if that’s not enough, I found it so bland and derivative that I fail to see how this book ever becomes influential.
• A Master of Djinn - Clark is amazing at worldbuilding, but one of his novellas should be nominated instead. I’d go for Ring Shout. A Master of Djinn entered the sub-genre of “public domain literary characters in alternate history” a little too late to influence it in a meaningful way, and made the main protagonist extremely unlikable.

To add:

• Too Like the Lighting by Ada Palmer - The entire series is amazing, and I’m shocked that it’s not being recommended more often. This book and its sequels may herald the return of more literary speculative fiction.
• Revelation Space by Alastair Reynolds - Just like Palmer channels Zelazny and revives literary SF, Reynolds channels Cordwainer Smith and introduces gothic horror in space to a new generation of readers. This book spawned an entire series, and I wouldn’t be surprised if other authors started mimicking his style.
• Leviathan Wakes by James S. A. Corey - If Relevation Space does not qualify (it was released in 2000, so technically in the last century), Leviathan Wakes and the series it spawned should take its place. The series is not high literature, but good, very accessible fun, which introduced loads of new reader to science fiction.