I've tried a few. And it's hard to get one that isn't too complicated.
People rave about Omnifocus. But it's WAY too easy, and I mean seriously easy to loose a job. Particularly if there's no due date or start date. Unless you go specifically looking for it under a context (Of which doesn't display numbers of jobs under them. If it did it might be easier).
ToDo isn't to bad, simple, bit easier, syncs with a lot of Apps, but I personally don't like how it looks.
Wunderlist did nothing but keep exiting for me on the iPad. So I haven't really used it that much. Which is a shame because it looks great. Has an App for each system. Windows, Mac you name it. Thing with Wunderlist too, is the Notes section. I'm on a bloody iPad. Give me some room for notes. Make it take up the whole screen while I'm editing. Would make it a lot better but at the moment it's like it's made for the iPhone and it's tiny.
RTM (Remember the Milk) Is quite good, it works well.
2Do is good, I like how you can have multiple calendars, password protect them. But it can be a little complicated, there's just massive amounts of options. Which is good, but can be daunting.
Things. I've used it for a while on my Mac, they've promised cloud syncing for an unbelieveably long time. It's apparently out soon, the cost is high, but it's one that I constantly use because there's just something simple and elegant about it.
Toodledo was alright too, but the layout needs some work.