Recently I was digging through the meeting minutesmeeting minutes for IAS and chanced upon a report prepared for Siegel to justify hiring him. The relevant pages are 13-18. In this report, it is stated that "we decided to propose at this juncture only one name, because it eclipses all others - that of Carl L. Siegel".
Later on, an excerpt from a letter from Courant is given which states "the only one of his generation whose strength could be compared with that of the mathematical heroes of the preceding era". More praise is lavished on by Chevalley, where he views Siegel as "on a level with a Hilbert or a Poincare" and by Hardy where he claims that nobody questioned if "he was the equal of any mathematician in his generation, and certainly I never doubted it myself".
With this in mind, it seems that it was in fact the consensus of the mathematical community then that Siegel was the greatest of his time, not just of Weil. Details of his accomplishments are given in the report for those interested. One thing I personally find puzzling is the lack of his influence in current areas of modern mathematics, as suppose to say Weil or Kolmogorov.
