A related question is "If I had the best professor in the world, what gems would they mention as examples of the beauty of computer science". These things maybe should be on the syllabus, but gems aren't necessarily seen as appropriate stepping stones in a learning process, and may only be accessible to the most capable students. Textbooks also, arguably, rarely offer charismatic enthusiasm or a glimpse of the sublime; they have to adopt a workaday attitude and play it safe.
Things that occur in books that might fit this bill of sublimity; Things that the very best practitioners get excited about:
Cormen names Tarjan's analysis of the complexity of the disjoint set union-find algorithm as his favourite thing[1]. Knuth is an appreciator of Tarjan's algorithm for finding strongly connected components[2].
Tarjan emerges as a contender for the algorithmicist's algorithmicist award.
[1] "It's not an algorithm, but a data structure. I've always marveled at the simple tree-based data structure for disjoint-set union, using union by rank and path compression (Section 21.3 in the third edition of CLRS). The code is amazingly simple, the data structure operations take just barely superlinear time, and the analysis (by Bob Tarjan) blows my mind."
[2] "While I was preparing for Volume 4 of TAOCP in the 90s, I wrote several dozen short routines using what you and I know as "literate programming." Those little essays have been packaged into The Stanford GraphBase (1994), and I still enjoy using and modifying them. My favorite is the implementation of Tarjan's beautiful algorithm for strong components, which appears on pages 512–519 of that book."
and
"The data structures that he devised for this problem fit together in an amazingly beautiful way, so that the quantities you need to look at while exploring a directed graph are always magically at your fingertips. And his algorithm also does topological sorting as a byproduct."
Finding this kind of enthusiastic recommendation is always a joy.