I have seen the 1960 winner "The Apartment" several times, just watched it a few days ago, in fact, on TCM. "The Apartment" is a really good movie, but "Spartacus" is much better, in my humble opinion. But then, as you said, they're very much different kinds of movies, so perhaps that isn't a fair comparison for that reason. I wouldn't have argued with it getting Best Picture in your Drama category, but "Spartacus" getting the award in another category. But I don't think "Spartacus" would have fit in any of the three categories you mentioned, so there would need to be another category. Historical Fiction, perhaps, or Big-Budget Costume Epic. I also think, incidentally, that the recent TV series "Spartacus" is the best TV series ever. In my humble opinion.
I also think "The Deer Hunter" is the WORST Best Picture winner ever. I saw it only once, in the movie theater when it was a current release. I'll never watch it again. The whole first half-hour of that movie was just a bunch of godawful noise, not exactly putting me into a receptive mood to be watching a good movie.
Yes, I think you're right that Best Picture winners often are not the best picture. But I also think they often are. This is a matter of subjective opinion, of course.
-- Ron