There are several good responses to 1 e4. Look at what the world champions played as black against 1 e4 and you will have your answer....
Good Response to e4
D5 is strong because you are threatening the white pawn and your queen is protecting your pawn. Also Nf6 is a good move if followed by black D5 but white can simply go to E5; threatening your knight and getting out of danger! Hope this helps!
I play e5. But I have seen an over abundance of Italian games lately, so I might be switching to something more exciting. Also, way too many people are trying to play the Lopez like Carlsen, now... also leading to not-so-exciting positions :(
Maybe back to the elephant gambit
c6, c5, d5, d6, e6, e5, g6, Nf6, maybe Nc6 and b6...
U can chose between all of these moves to answer 1.e4, and in each of them u have many viable alternatives...
The better move will be the move u like and know more...
A seriously underrated opening is 1..Nc6. If white plays 2. Nf3, black can always play 2..e5 if he wants bad transpose back into a king pawn game.
Try the Scandinavian Defense: 3...Qd6 Variation, aka Gubinsky-Melts Variation. For the inside story join us at http://www.chess.com/groups/view/scandinavian-defense-3qd6-variation
French Defense is very good for black. So long as you don't accept any gambits you should equalise the game fairly quickly.