It's funny because a lot of the time, I tend to cling to the first album that really sold me on a band (in this case, Spirit the Earth Aflame). But over the last couple of years, I've listened to a ton of Primordial and I honestly think The Gathering Wilderness could be their best effort yet, followed closely by A Journeys End.
Overall though, TGW, StEA, and AJE are all MUST HAVEs (the difference between the three is almost negligble). Everything else is secondary.
Great band and pretty much the core reason for me flying to Heathen Crusade.
Similar Artists: Nobody really, but check out Destroyer 666 and Drudkh's Autumn Aurora (all extreme vocals however). Of course, Nemtheanga is a huge selling point for Primordial so you might also want to listen to Void of Silence ("Human Antithesis").