I confess that....I may very well have a different "ranking" when it comes to the major Oscars.
Traditionally, "Best Picture" is considered the top prize overall. In some years, "Best Screenplay" has been viewed as almost a 2nd place. For me, I think it might actually be the reverse. I tend to like strong, plot-driven movies that rely very heavily on substance and much less on style. Flash and color and the like are all fun, but a strong story wins me every time. I think that may be why I've often picked a screenplay winner as my personal best picture. That's what "works" for me.
I wanted Milk to win last night because of the story and the acting. It grabbed me and I was lost in it...almost forgetting it wasn't more of a true documentary. But, I can admit it wasn't too much for the eyes. The visuals fit the story, but they didn't go beyond it. Slumdog was much more visual...and musical too. Maybe that's part of the difference...a "Best Picture" has more of a balance between plot, acting, visuals, and music. It is a package deal. I'm more of an elements girl.
I remember many moons ago my college roommate and I were debating over email with my cousin about Titanic vs. Good Will Hunting. I still prefer Good Will Hunting, by far. But it is definitely a "simpler" movie in terms of style. So maybe my diagreements with the Academy become an "It's not you, it's me" thing.
Or a difference in priorities.
Or, maybe I'm right and they're wrong...just 'cause. :P