Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
freemium $12.5 / Monthly (Per user - Business Class)
Trello.com - the big fish. All kanban, very focused on individual boards but also very easy to use for newcomers. The power ups make this infinitely usable, and with zapier integration, it's pretty unstoppable.