I'm considering pitching switching to Bugzilla again, I'd love to use JIRA but we're all about the open source where I'm at. I'll have to play around with Mantis (never heard of it). I tried getting a Scarab install setup but I struggled with it. I think Bugzilla would be good since pretty much everyone knows how to use it.
I'd really like a simple one but simple ones don't have all the features. My gripe with Bugzilla is that it's overkill I think.
A good tracker has always been on my list of projects to do. I might give it another whirl using Grails as a pet project.