King Pawn play

Is e4 the best opening in chess?  And what is the best reply to e4?

Yes.  It's certainly the most ambitious.  1...e5 is the "best" reply. 

Well there is no "best opening" as has been mentioned many times on chess.com. Some people like e4, others d4. c4 and Nf3 are also very popular first moves. And as far as e5 being the "best" response to e4??? Well that depends on how you define best. e5 is best for a beginner, but there is a reason why c5 is the most popular...

e4 is not the best opening.  c5 or e6 are the best replies.

I think you'll find at the very top of the game 1...e5 is currently most popular.  Those that play 1...c5 do not do so because it is best, but for other, practical reasons.  You're correct though, there isnt really a truly best opening because it's only the first move, and anything sensible should be just fine.  Still, 1.e4 best by test.  Also best by the phenomenal number of things it accomplishes. 

Well c5 is only popular because of the Sicilian Defense; which is an old favorite of our masters from yester year.  The e5 idea seems to offer black the same chances as white and therefore is quite drawing.  The Fenchy e6 can lead to more dynamic play; but be careful, on both sides.  I think e4 is only good for quick castles--and of course this is good and bad.