Metallica
That was my initial gut reaction. But as I thought through it, I don't really view what they've done in the last 20 years as very good or groundbreaking in any way, shape, or form. To me, there was a serious dropoff in quality after the Black album, and one really solid album in the last 20 years doesn't earn them "best" by any standards.
Honestly, I would have to go with DT. They obviously have not had as huge an influence on the metal scene as a more mainstream metal band. But the sheer volume and quality of music they've put out in the past 20 years destroys what anybody else has done.