If you had to pick one, I'd say the single most important high level math topic that'd be useful for computer science is Graph theory. There are so many applica

If you had to pick one, I'd say the single most important high level math topic that'd be useful for computer science is Graph theory. There are so many applications of graphs in computer science. The thing about math is that regardless of which field you work in the treatment is the same. You start by stating a set of axioms. Based on these axioms you set up definitions and then prove lemmas, corrolaries and theorems. This approach isn't really necessary or directly useful for computer science though. But it is very beautiful. For example, if you truly want to appreciate, in it's full force, why a player playing a fair game against a casino will always eventually lose (theorem called Gambler's ruin) you can study stochastic processes with full rigour (this is also a very useful area). But that full rigour isn't necessary to not blow your savings on games of chance :)