Todoist is hands down my favorite. It has integration with lots of other software (Gmail, Outlook for Windows, Postbox, Chrome, etc..). Additionally it has convenient quick keys for adding a task to your inbox without having to switch applications. I find their karma system enticing. I have no affiliation with the company, just a huge fan of their work. I was on a Wanderlust kick for a while, but their reliability has been troubling.