Hey Vicky,
It really depends on what you’re looking for in a project management tool-they all offer different features that might be more or less adapted to your workflow. Some of the most popular/robust ones include Trello, Asana, Jira or Wrike. Here’s a resource: Seven Of The Best Project Management Tools For Your Business
Internally, we use a combination of those tools and connect them together via our own app 
I work for Unito, and we’re developing a Hubspot connector for those tools. You’d be able to integrate tasks bi-directionally with Hubspot. We’re releasing the free beta version soon, so once you choose the right project management tool, you can sign up to test the integration with Hubspot here.
Let me know if you have any questions!