>>12428242Abstract algebra (from groups to galois theory) is invaluable in theoretical CS.
Analysis is becoming really popular, especially functional analysis for dimensionality reduction and low distortion metrics.
If you name me a field of mathematics, I can likely give you a well motivated and funded part of TCS that uses it, but it's really up to your tastes / background at that point. Complex analysis shows up naturally because of generating functions and harder asymptotics, but not everyone does that. Topology shows up everywhere in computational geometry and scientific applications, but not everyone does that. Logic has a large place in CS of course, and there's more theory of countable structures (ie classical recursion theory) that's in CS than there has ever been.
I'd suggest just doing honors and grad classes in the core mathematics like algebra, analysis, and topology, and then branching out after you hit that point.
