Actually I would say NZB, as most torrent releases are uploaded in this method quite fast and you don't have to make sure that you keep your ratio and instant full speed. But strictly talking about torrents, private are good for fast speeds and security but in general suck for seeding. Public torrents for the older stuff (eg tv shows or movies in Dutch). Most negative of public torrents is that in general they don't have the speed of private torrents. Recently I bought a NAS (Network Attached Storage) with embedded Utorrent so I don't care about speed or seeding because it is separate from my computer so it downloads and seeds 24/7.
So there is not 'best' as they all have their advantages (and disadvantages).