OK on the Sabbath, but I'd go for the fiver and add "Sabbath Bloody Sabbath" as well.
Zeppelin? The first four albums for sure.
Rolling Stones? "Beggars Banquet" through "Exile" works for me!
I'll add Joy Division, even though they only made two proper albums- "Unknown Pleasures" and "Closer." But those two are still better than a lot of folk's best three!
And um, how about ol' Bob Dylan? "Bringing It All Back Home," "Highway 61 Revisited," "Blonde on Blonde," and "John Wesley Harding" all qualify as essential- and "Blonde" is a double, so Bob gets a fiver! And more recently, "Love and Theft," "Modern Times," and "Together Through Life" make a solid three-pack.
Jimi Hendrix!!! The three studio records, through "Electric Ladyland" (another double!) are first-rate all the way. For that matter, so are the posthumous "Cry of Love" and "Rainbow Bridge" albums. Which leads to another category: largest number of great albums by deceased artists? OK, maybe we won't go there...
New Order- "Power Corruption and Lies," "Low-Life," "Brotherhood," and "Technique" are all solid work.
If we wander over to the Jazz aisle, there's a whole slew of folks with long winning streaks- Coltrane, Andrew Hill, Grant Green, and of course Miles Davis, off the point of my head...