Egan is a close second for my all-time favorite sci-fi author. Some of his books read like mathematical dialogues or transcriptions of lectures (lots of the Orthogonal Series is conversations between professors and their grad students reasoning about physics of a world that we don't live in), but he also delves deep into mathematics and fringe ideas and presents them in a vista where you can enjoy the absolute splendor of the abstractions.

He also never thinks small. I thought I knew "the point" of Diaspora about 6 or 7 times, but then he just "zoomed out" and made the last point look small and trivial by comparison. The opening chapter of that book is so abstract and yet describes the birth of a consciousness in a way that feels understandable, believable, and internally consistent.

If you love finding interesting puzzles to reason about, then I strongly recommend Egan's books!

I haven't read Permutation City, though. I'm bumping this one to the top of the queue :)