OmniFocus 2
Task management is a fairly personal thing. What works for one person won't work for another, but OmniFocus has been working well for me for years, and I've been using the v2.0 betas for so long that I feel like I must have already paid for it.
I can't recommend it universally, but it's definitely one of the contenders, and if OmniFocus seems like it might be your thing, this is a great time to give it a try, and see if it's a good fit for you.
Shawn Blanc's Preview of OmniFocus 2 is good reading, if you want to be persuaded.