It would have to be Todoist.
It's my favorite because I love how simple it is. It has a very simple interface, with clean lines and a lot of white space so you won't feel cluttered, even if you have a hundred items in your to-do list. It has all the basic features of task management app, without the bells and whistles so you won't be bogged down by unnecessary features. And I love how it integrates with all the other apps I use.
I also like how the app gamified productivity with its Karma Points. Because I get points whenever I cross out items on my list and lose points when I don't, it keeps me motivated to finish my tasks. I want my points to jeep going up, after all.
But, most of all, it's my favorite online productivity tool because it keeps me productive without a lot of upkeep. I just list all the tasks I have to do, add due dates and that's it. Sure, I can organize my list to death, but I don't have to. It can work with even with the simplest workflow.
My favorite is that there are two online applications that I play faithfully today, steemit and kwai goe. And now steemit is now very weak, but I still be patient and always struggle. What are you doing?