I'd suggest checking the respective web sites. The IBM web site will probably show that DB2 is clearly superior, and the Oracle web site will probably show that Oracle is clearly superior. You might find a comparison on any of the websites that rank/compare products, and that will show a different slant on the same data.
The biggest problem is that "best" is a very subjective term. One person's best is not necessarily another person's best because they will probably have different needs, and the more different their needs, the more different their opinions about which product is best.