Depends on what your definition of 'best' is. Lot of users here will vouch by in-built task list - I personally used it for a long time. I found a new task list called maratick (freeware) that I like better than the inbuilt one.
You can try it from following web site: Ted Regsuren's BB page
Windows Mobile gang vouches by ListPro that is not available for Blackberry (yet).
Be your own judge in choosing what suits your needs following testing.