As someone who has read all three (Illuminatus at least twice, Foucalt's Pendulum probably 4 or more), I agree.
Da Vinci Code is a dim shadow of them both.
Not sure what else /would/ compare to those two, really.
@CoronaCoreanici @emanate @Azure
Actually I suppose Thomas Pychon's books largely count, at least if we're going with both complexity and conspiracies-tying-into-actual-history.