One of my pet peeves with Sabbath is the imbalance of perception. To classic rock fans, Sabbath ends in 1978. To old school metal fans, Sabbath exists between 1980 and 1982. To the hipsters, only the Martin-era albums deserve attention.
It's true that the albums from the individual eras of the band sometimes sound radically different in tone, style and atmosphere, but I still have no problem accepting for myself that Paranoid, Heaven and Hell and Headless Cross are stepping stones in the same band's career - even if there is sometimes only one person to bind it all together. So all eras deserve the same recognition.
Black Sabbath is probably my second favourite metal band after Maiden, because I don't consider anything they've done a total failure (unlike, say, Judas Priest) and even their worst moments blow away very much out there. Sure, there are some low points in their discography, like Never Say Die or Cross Purposes, but even they have a lot on them that I enjoy. Even though I consider the Dio-era lineup to be among the very best things that ever existed in metal, I can by no means reduce the band's output to that. So I have severe difficulties answering the poll's question. I guess if I really had to boil it down to five, it'd be the debut, Masters of Reality, Heaven and Hell, Seventh Star and Headless Cross. Runners up would be Vol. 4, Mob Rules and Tyr.