We use Atlassian Jira currently, mainly due to the ability to stack with Confluence, BitBucket etc and extend its functionality with plugins. Plus it's got all the tools needed for Scrum or Kanban depending on which you prefer.

I use Kanban because... it's simple. And easy. And I don't like wasting time on understanding other complicated tools. ---------- EDIT: I forgot to write the name, it's Kanban Tool, I use it for several years (I resigned from Trello).