Exactly. I like Sabbath, but they are nowhere near a favorite of mine, largely because they are so heavily blues influenced. I don't like rock or hard rock, because it is basically modernized blues.
Well, Black Sabbath is probably my all-time favorite band, and I'll admit that a lot of the appeal comes from their use of the blues scale. I think many of the catchiest, most pleasing, and most powerful riffs in rock/metal rely on the scale -- and Black Sabbath were masters at exploiting it.
If you're judging them based on their early albums, you might want to try
Sabbath Bloody Sabbath and
Sabotage, as these are generally more 'melodic' than the early stuff. Of course, they're also more stylistically eclectic and less heavy, so that might be a minus for you.
I can understand why they might not work for people who are into more melodramatic, chaotic, or 'busy' metal. Sabbath had a pretty laid-back and simplistic sound (most of the time), and that sound just clicks really well with me.
As to whether they deserve to be called the best metal band ever: you do have to give them credit for revolutionizing rock music, and for the absolutely massive influence they had. And even if you don't like blues-based music, it's hard to deny that they wrote some fantastic, memorable riffs -- and lots of them -- so I'd say their songwriting talent is at least above average.