Bug Tracking Database Systems?

Jeremy asks: "I am looking into changing our bug tracking database. We are using Gnats for the time being, and it doesn't easily offer information that will show statistical trends such as the number of bugs for each week over the last 10 weeks, or the number of bugs each developer has fixed for each week over the last 10 weeks, and so forth. What, in you opinion, is one of the best bug tracking software in the open source world?"