I just use todoist. Very simple, you can create a bit of structure if you want it, or just keep it very simple as a to-do list & color code priority.

The primary downside is that viewing history (closed tasks) is complete shit. They have an API, and that's the only way of getting history out in any sort of convenient format to review. A very strange oversight.

I'm also not sure how well it would scale to large teams.