My thesis advisor was Paul Conrad, who did much to create the theory of lattice-ordered groups.

His thesis advisor was Reinhold Baer, one of the great group theorists of the 20th century.

His thesis advisor was David Hilbert, who made fundamental contributions to many areas of mathematics, including invariant theory, geometry, algebraic number fields, functional analysis, integral equations, mathematical physics, and the calculus of variations.

His thesis advisor was Hellmut Kneser, instrumental in founding the Mathematical Research Institute at Oberwolfach.

His thesis advisor was Ferdinand Lindemann, famous for first proving that pi is transcendental, and that consequently it is impossible to "square the circle".

His thesis advisor was Felix Klein, the great geometer who established that non-Euclidean geometry is consistent (assuming that Euclidean geometry is), and promoted the Erlanger Programme, defining geometry as the study of the properties of a space that are invariant under a given group of transformations.

His thesis advisor was Julius Plucker, who did important work in both geometry and physics. Plucker's advisor was Christian Gerling, whose advisor was Karl Gauss himself!