I think they are obviously all bright in different ways of course.
Fred, as stated many times previously, is amazingly well with science and it just seems to come natural to her.
But Willow is more of the liberal arts (history, etc.) and the best all around with computers (Her hacking was pretty astounding.). And I believe she graduated with a 4.0 and got accepted to every college in the U.S, not to mention some in Europe (Oxford, aka Giles' land) and I believe she scored a 1580 (This is because I remember her complaining about a 780, so if she complained about the 780, wouldn't the other section be perfect? My assumption.) Not to mention her knowledge of magicks, since she is the most powerful witch on the planet!
Although you have Giles, who is probably the most well rounded of them. He attended what is probably the finest of schools in the world (Although Willow could have and UCLA is decent, they pale in comparison to ole Ox. Willow should have went somewhere better, and I wonder where Fred did her undergrad work at?). He can speak a plethora of languages, knows a lot abot history and demons and I bet he's probably not that bad at mathematics and science.
So Fred vs. Willow, Willow because we've seen that she can excel in all subjects, but I personally have only seen Fred excel in her main areas.