Most consistent discographies in Metal?
I would say try to find bands with a higher quantity with also quality. Judas Priest is a good start.
I think of Priest as more of a "high highs" and "low lows" kind of band, but when you've been around as long as they have there's bound to be classic tracks for days.
"And all the pressure that's been building up
For all the years it bore the load
The cracks appear, the frame starts to distort
Ready to explode..."
I mentioned this earlier but I find a lot of German metal bands to be bastions of consistency. Rage, Stormwarrior, Accept, Grave Digger, Kai Hansen on vocals era Gamma Ray, Iron Savior, Running Wild. The Germans just don't stop!
Other bands that usually deliver: Falconer, Immolation, Persuader, Sabaton, Bolt Thrower, Marduk.
A lot of those bands definitely can be accused of making the same album over and over again, but in the immortal words of Yngwie: "How can less be more? More is more!"
Now get listening!
For me this is THE most legendary run of albums by a metal band.