For example, (since I don't really have much time)
1. Topology (book by Munkres)
2. Real Analysis and Measure Theory (book series by Stein Shakarchi)
3. Algebra (book by Aluffi)
4. Linear Algebra (book by Friedberg Insel)
5. Measure Theoretic Probability (book by Cinlar)
6. Differential Geometry (book Smooth Manifolds by Lee)
7. Numerical Analysis (book by Quarteroni)
8. Set Theory and Propositional Logic (books by Goldrei)
This is what one will mainly learn in a strong undergrad/grad math program. Once this is done, then there are different tracks to follow.