Blass-Debrunner-Funk is the absolute best reference grammar for Greek. However, for learning Greek, most seminaries these days are using Machen's book, now in a revised second edition. However, I do not like Machen. I think that in his desire to teach New Testament Greek, he avoids introducing some of the rarer forms. If you are really going to learn Greek well, then you need to learn classical Greek. NT Greek will then be as easy as falling off a log. For classical Greek, I highly recommend Anne Groton's book From Alpha to Omega, published by Focus books. This is an extremely well-laid out book, and prepares you to read anything you want in Greek. After all, though the NT is the most important thing to read in Greek, it isn't the only thing worth reading in Greek.