Choosing the right bug tracker requires that you know who is going to use it (and how it is going to be used). I've used Bugzilla and Mantis and found Bugzilla better from a technical point of view but Mantis wins if some of your bug reporters are not programmers / not programmer minded. Its interface is less 'threatening' for a novice bugtracker user.
If you are going to have a private bugtracker you also need to consider the options it gives you to specify who is allowed to view/edit etc.
