If we except the great name of Newton (and the exception is one that the great Gauss himself would have been delighted to make) it is probable that no mathematician of any age or country has ever surpassed Gauss in the combination of an abundant fertility of invention with an absolute vigorousness in demonstration...
Henry John Stephen Smith as cited by the School of Mathematics and Statistics at the University of St Andrews in Scotland (http://www-history.mcs.st-andrews.ac.uk/Biographies/Smith.html).