Material packages tops our list.
We can build anything we want in a reasonable amount of time but coming up with the perfect material is by far the most time consuming vampire as part of the build process.
Animation packages, in FBX source of course, would be second on the list.
Overall common build elements sorted into share based folders easy to catalog. The most annoying thing about asset collecting is not knowing what you already have or browse a properly formatted content folder.
If we can buy something that does not require reinventing the wheel the more the better.
An Internet based asset management application would be nice as well. Daz Studio for example would be a perfect solution if it could access and make use of an Internet connected asset server.