Well, it depends...
Young of the year crappie can do surprisingly well without a lot of cover as they are mostly pelagic during their early life feeding upon zooplankton in open water areas. Young of the year crappie may use cover but will quickly head to open water if predation pressure from largemouth bass or other predators increases.
The 'best" cover may be determined by numerous factors. Vegetation can supply superb nursery habitat but waters that are extremely turbid or waters with extremely sharp shoreline slopes can inhibit vegetation. Some vegetation establishment projects in Kansas waters have been quite successful in providing increased habitat for YOY fishes.
Availability of a certain type of cover may make it the 'best'. Abundant usually equates increased availability and decreased cost. A study in Kansas showed that readily available cedar trees produced good habitat at a reduced cost. The trees were easy to get, often free from landowners, and created good fish habitat. Other tree species, or a combination of species, may provide better habitat but increased cost to obtain or transport from longer distances would quickly outweigh the benefits. It takes a lot of material to make an impact on even a 200 acre lake so it is usually necessary to keep prices down to allow for the addition of the large volumes required.
In Kansas lakes, we usually don't have a problem with too few crappie in the small lakes but rather the opposite and we end up trying to control the over abundant crappie populations instead of trying to promote them.
I know I haven't answered your question very well, but the best cover may vary between individual lakes and locations depending on a wide variety of variables.