I would say fogbugz. It all depends on cost though. Most of the products/apps mentioned in the question are free or open source. If cost is a problem then one of those apps would be great.
The other nice thing about fogbugz is they can host it for you. For such a small team you may not want to worry about hosting the other software products yourself and making sure they are backed up and all that.
Just things to think about. I have tried most of those products and settled on fogbugz in the end. I am also a part of a very small team and it worked great.
