Phabricator looks cool... a little OT but I love the footer links, hah!
I've used a number of systems over the years, and frankly the success of each of these has had a lot more to do with the people involved than what software we used.
Git has probably been one of my favorite tools, historically speaking.
But when you spend more time managing tools and tracking changes than you do actually developing... there's a problem, hehe.
Call me primitive, but most of my development is independent these days so email is my #1 project collaboration tool!