I've been using Sentry for the past few years and have found it to be the best bug tracking tool in terms of user experience. I think, what I like most about, is definitely UX and the versatility of integrations it provides.
Bugzilla
> and why do you prefer it?
Probably familiarity more than anything. But it's Open Source and it "just works" so I don't feel compelled to spend a lot of time evaluating alternatives. And of the alternatives I've been compelled to use at one time or another (mainly Jira) I was not impressed.