Yaron Minsky

Trello is nice for simple workflows. Basecamp works, for projects that require more asynchronous communication and coordination without forcing any workflow on you. Jira works, for quickly frustrating a team and making them reconsider their career choice.

Curious to know if you've tried @GetZepel. It lets you view and plan your project at a system level and ultimately track it at a low-level, at a specific discipline, or as a whole functionality.

Hey, don't call me that.

basecamp. it's really good and now free for individual use.

How complex? For task prioritization and simple workflows, Trello is quite nice. If more communication is required between the team, designs and so on, Basecamp is great.

We made Redmine 3.4 our home. It may seem "old school" but it's quite easy to customize for a variety of workflows (moreso in my opinion than Request::Tracker, our former choice).

Jira

Nowadays we use Favro. It allows you to build various levels of structure, but doesn’t force any particular way of organising your tasks, so you can probably adapt it to your existing workflow.