My personal favorite is Blindsight [0][1]. I read it multiple times (3 or 4, I think) over the years.
If you like Blindsight, you’ll probably want to continue with the sidequel, Echopraxia. You can also find them bundled together under the name Firefall.
And I think out of his main works, Blindsight is probably the most accessible. It also led me to Thomas Metzinger’s “The Ego Tunnel: The Science of the Mind and the Myth of the Self” [2], which Watts used as a source material in his research and had good things to say about.
The Rifters series is great, too, but there’s a sort of darkness or heaviness to it that may not work for everyone.
And bonus content, a fan made short movie based on Blindsight, here: [3].
I hope this helps. :)
Links:
[0]: https://en.wikipedia.org/wiki/Blindsight_(Watts_novel)
[1]: https://www.rifters.com/real/Blindsight.htm