I have to admit - I really like Any.Do (http://www.any.do/). It's clean, the interface is easy, it's functional...but I don't use it.
Because Trello just offers me more.
In a nutshell, Trello is a to-do list that can also function as a planner, organiser and project management tool. Within 'boards' you can create various lists and within those lists cards. The cards can be simple, one-item notes (like a standard to-do list) or by clicking on the card you can also access embedded functions within the card, such as checklists, notes, descriptions and more.
We use it at HPN for our individual to-do lists and also as a visual planning board for course development, event management, and we've even begun to use it as a Wiki. We're there (in Trello that is) so often, that it makes sense to have it be our 'go to' place for all things HPN, especially as we may have documents and notes spread out all over the place. Trello gives us a one-stop shop to find anything that we need and gives us the ability to move cards to other members of the team.
Trello has integrations for Gmail and Chrome too so that you can make notes and add tasks for you or your team members on the fly.
What's your favourite to-do app?