Could you say whether you are willing to pay (FogBugz) or not, in which case it gets interesting...
I always used Bugzilla when someone else chose the problem tracker, but when I got to choose, I chose Mantis. Why? Simply because Bugzilla is written in PErl and Mantis in PHP, whcih I prefer.
Of course, you may not want to hack the code, in which case, decide which featres are important to you ... and 'll find that most problem tracking systems support them all.
Quid nunc? Well, a tool on its own is just a tool. And while all speak of a toolchain, most just mean a loose collection of tools. Why not look for a problem tracker that "plays well with other children"? That is to say, interfaces well with your IDE, your build tool, your version control system ...
In fact, I think I'll go now and ask a question about the best linked toolchain ...