Well then I am afraid that you need to get out more. No denying their impact, nor their creativity or skills, but at least for me, they are not "Best of All Time" material.
But it really all boils down to personal taste. I can't really critisize you for saying they are, because they really helped pave the way for bands that I have grown up listening to and have had a profound impact on me. They are also good in and of themselves. I even had a period where I was really into Sabbath and listened to their stuff pretty much all the time. But now listening to them is more like being reminded of a fond memory than actually appreciating the music. It's hard to explain.
Now for a band that would fit my own criteria...Emperor is really the only one I could sit and justify. But even then it would only be two albums. With Dodheimsgard, they have one album that is just imo a pure creative masterpiece, whilst the rest of their stuff is pretty much run of the mill, except the last album which was crap. Origin is another band that would fit, but saying a death/grind band is the best metal act of all time is doing a diservice to the genre.