...whom would you choose? It doesn't matter whether the quotes are serious or comical, profound or simply delightful, as long as they bear significance, to those who might hear the speaker or to you personally.

For me, those would be Stephen Fry and Neil deGrasse Tyson. The former has put up a significant effort to defend the decency of humanity (his blog post on the Charlie Hebdo terror act alone puts him high on a ladder, in my book), while the latter did the same for curiosity, discovery, truth and science as opposed to ignorance.