



I would have to say that the "greatest philosopher" title would have to
go to Plato since the whole enterprise of philosophy was essentially
defined by him.  Although he got most of his answers wrong, he did 
definitively identify what the important questions are.  I think it
was Descartes who said that "All philosophy is just a footnote to Plato."

If I were to choose which philosopher made the most important advances
in human knowledge over his lifetime, that's simple...it is Aristotle.
This is so much the case that many simply refer to him as "the philosopher".

Regarding Nietzsche, he's one of the most entertaining, although since his
ideas were so fragmented (and since his life was cut short) it is doubtful
that his influence as a philosopher is likely to be very extensive 500 years
from now.  They'll probably still be reading him in 500 years though.

As for "modern" philosophers, I would have to say that Kant was the most
influential since he had such a strong influence on almost everyone who
came after him (and unfortunately, they maintained his errors and 
amplified them over time).

I would say that the most influential "american" philosopher would have to
be Dewey.

But as to the question of what philosopher will be most highly regarded in
500 years, it may very well be Ayn Rand (who in every important respect
was "American", but was born in Russia).  But I guess that remains to be seen.