the fact that the only 20th century mathematician mentioned here is erdos, suggests to me that the authors of these opinions do not know anything first hand about (20th century?) mathematics, but are only parroting what they read in the popular press.
essentially anyone in the mathematical community would have mentioned poincare, lebesgue, weyl, weil, mumford, thom, jones, bott, hironaka, wiener, hopf, artin, artin (yes, there are two of them), oka, grauert, lefschetz, hilbert (at least his famous congress talk was in 1900 and guided much of 20th century mathematics), zariski, cartan, enriques, serre, morse, atiyah, grothendieck, cohen, deligne, bombieri, brauer, igusa, fulton, chow, harish - chandra, kodaira, chern, tate, shafarevich, kontsevich, manin, washnitzer, witten, mori, sullivan, etc etc etc...
are these names known to readers of this site? if not, they qualify for unsung heroes, in the sense at least that the general public does not know who they are.
i recommend also to fans of gauss that, if they have not done so, they at least read some of his work "disquisitiones arithmeticae" (available in english translation), and to adherents of galileo that they read his "two new sciences", which is much easier and reads almost like a socratic dialogue. it is very striking that he obtains his results without even a decent notastion for numbers, representing each real number as the ratio of a pair of straight lines!
galileo shows for example that a projectile moving under the influence only of gravity travels in a path shaped like a parabola, ASSUMING THAT LINES DRAWN TO THE CENTER OF THE EARTH ARE PARALLEL, which he also observes is not quite true, since they meet at the Earth's center. How many modern calculus books bother to point out this fact, before proving the otherwise false statement above? This shows the difference between reading the masters and their pupils, as abel put it, since regardless of their limitations in technique the old masters are possessed of amazing amounts of insight.
How many people are aware that galois last letter contained more than group theory and its application to solving equations? (the second half being an anticipation of riemanns theory of abelian integrals, including i believe the concept of the genus). you can only know this if you read it, and do not depend on textbooks (or websites) for an account of "galois theory".
huff puff