Here are my reasons why:
1. Rob Halford has an exceedingly immense vocal range. His metal scream is the best in all of metal, and he has an incredible talent to sing in many different moods and tones, and is not limited to sounding the same all the time.
2. Judas Priest was the first rock band (that I can think of anyways) to have two lead guitar players, both playing in beautiful harmony with each other.
3. I've seen best metal albums lists and out of all the bands listed Priest was listed the most. They have SEVEN albums on the top 100 best metal albums list.
4. They've been around since 1969 and they still tour to this day.
5. They never run out of ideas. You've never heard Judas Priest play the same song twice on any of their albums. This is a great achievement seeing as though they've released 16 studio albums in their career.