Maybe not milestones, per se, but Nevermore Politics of Ecstasy, Dreaming Neon Black, and Dead Heart, in a Dead World is a nice run of quality releases.
Forbidden has a similar stretch with Forbidden Evil, Twisted Into Form, and Distortion.
Anekdoten's entire catalog is top notch, so that's 4 consecutive excellent releases, but maybe not groundbreaking.
Ok, here's a nice run:
Rush - 2112, A Farewell to Kings, Hemispheres, Permanent Waves, Moving Pictures
Nobody's touching that, and you could arguably extend it a couple albums in each direction.
Oh, and Thought Industry, all 5 of them are extremely unique and excellent.