The API and ILSAC specs are quality floors--and who's to say that every part of every batch meets spec? They probably meet spec on average.
Some oils are made better for competitive reasons. In 15W-40, Schaeffer Moly Bond X-200 is clearly superior to other conventional oils.
In passenger car grades, there might be some smaller outfits with premium products...I don't have brands at hand. Among the biggies, I think Johnny has said that Shell positions their products in this quality order...Pennz, Shell, Quaker State. I've had excellent results with Chevron. I'd look to Chevron, Pennz, BP's Castrol, Mobil.
I use Schaeffer's syn blend oils with 10,000 mile oil & filter drain intervals, get excellent lab results, and have a lower oil & filter cost per mile than a cheaper oil with 5k ODI.