Nigel Kalton (1946-2010) was an extraordinary mathematician who made major contributions to an amazingly diverse range of fields over the course of his career.
Fritz Gesztesy works in areas of Mathematical Physics with emphasis on Spectral Theory and Differential Operators.
Gilles Godefroy works in functional analysis, and particularly in geometry of Banach spaces.
Loukas Grafakos works in the area of Fourier Analysis and Applications.
Igor Verbitsky works in the areas of Harmonic Analysis and Partial Differential Equations.