In your opinion, what is the best defense against 1.d4?

There is no single best defence. It depends on your own style.

I used to play the Dutch exclusively as a reply to d4, but now I have changed to the Nimzo-Indian (or the Queen's Indian depending on white's responses). The Slav and Semi-Slav are extremely popular and solid. The Grünfeld is exciting and ambitious, but can be a bit dangerous. The Benoni has some serious theoretical problems at the minute that haven't been answered satisfactorily. The QGD and QGA are classics that still have a big following. The King's Indian has had phases of being in and out of favour as theory develops - but you need to know loads of theory if you really want to play it successfully.

All openings have good points and bad points. You just have to weigh up which combination of good and bad suits you. The Nimzo is cool because it gives black a good fighting chance without conceeding too much. It has stood the test of time and is still played frequently at the highest level. I have looked for an alternative to the Dutch because it does introduce some weaknesses into your own position, even though it often leads to exciting, attacking games. I'll be using it as my second option in future.