There isn't really a "best" one and the statistics used to determine the most successful openings are based on titled players. So you should just play whatever you want... But I think the Nimzo-Indian, Slav and Grunfeld are the most popular right now. I could be wrong though.
Black response to d4

I prefer 1...Nf6 and go into the Nimzo or Queen's Indian:
1.d4 Nf6 2.c4 e6 3.Nc3 (3.Nf3 g6) Bb4
They're very similar and you can use the same general plans and principles. At your and my level of chess, you shouldn't need much theory.
Can someone simply list the two or three best systems for black against d4? Thanks in advance.
Bluesman.