According to Hans Berliner, correspondence chess champion, Bg5 is the best.
From his book "The System. A World Champion's Approach to Chess" he stated, and Fischer agrees, that Bg5 maintain the most amount of pressure. Nf3 is not as good, because you has three major plans in the QGD exchange variation and only one has the knight on f3.
My opinion is that Bf4 is stronger below master level. I like the idea of Bxb8, not having the question put to the bishop on g5, and it reduces the possibility of Bd6. There are disadvantages to putting the bishop of f4.
The answer depends upon what type of positions you like to play. For positional games, the knight goes to f3, for center control and kingside attacks, the knight goes to g3 via e2.