Best is to be determined only by you. What is good for me, might not be good for you.
Where do you plan to use it? Do you have a BB data plan? Have you done any research on your own?
read the yellow sticky at the top of this section. It has reviews of most of the available map applications for the BB.
For me, here in the US, I use Google Maps as it fits my needs the best. To install, use Google search and download to your device. MUST have a BB data plan however.