I've been a big fan of Bob the Monkey or the Cow, depending how you look at it. All my to-do items are tracked withing remember the milk, nicely organized in lists, shared among people, and tagged with something meaningful. The user interface has been the same for a while and can be improved upon.
A recent blog post about Gmail tasks triggered reexamining this feature. While promising, it falls short because it lacks the following features I am used to (correct me if I overlooked how it works with Gmail tasks):
- ability to specify a due time and an estimate how long a task will take.
- ability to specify recurring tasks: e.g. pay PG&E every 12th of the month
- ability to share tasks with others
- ability to add a location and map to a task
- ability to add a priority to a task
- ability to add tags to a task
- ability to add contacts to a task
- smart lists : e.g. show all work related (tag=work) items due this week
- google.com/tasks should provide a great user interface to sort, rearrange and do bulk operations on tasks