I've been using Flyspray for several years so far and have found nothing better.
Flyspray provides everything I need, starting from a powerful ticket system with the support for a wide range of parameters, to the easy-in-use control panel. It can handle multiple projects in the same system, each with its own custom settings and parameter types, such as ticket type and category. I especially enjoyed the permission system, with the support for anonymous tickets and per-project settings. Another advantage is the user interface that is elegant and functional.
The only thing I miss is the text formatting system. The tickets are displayed in the plain text, whereas I would like to use some form of formatting (even HTML). However, it is not a very big disadvantage, as the plain text can be also well-formatted and clean.