Combinatorics is some classic computer science-ish material, showing up in a lot of Knuth's writings.
The other topics may well be useful for specific applications, but having no idea about what you intend to do with this knowledge, I would vote for combinatorics as "most useful to a programmer".
The other topics may well be useful for specific applications, but having no idea about what you intend to do with this knowledge, I would vote for combinatorics as "most useful to a programmer".