I love outliners in general, but in particular I love NeO. For a while I toyed with TAO, but I could never quite get my head round it, then I used OmniOutliner, but somehow I was always hitting serious limitations. Tree is very nice and simple, but also limited, likewise Opal. Deep Notes too.
NeO is a reborn TAO, and no simpler, but I spent some serious time trying to understand it, to the extent that I now find it actually simpler to use than OO, as well as being much more powerful and flexible. It has a wealth of unique features, such as split editing, scaling, links and clones, and just as important from my viewpoint, it gives the user minute control over every aspect of the appearance of the outline: they can look as simple and uncluttered as you like, or they can be loaded with columns and metadata.
The extra power does come at the expense of complexity: it takes a while to learn, and it is not yet bug-free, though I have found it perfectly useable. Talking of expense, the most amazing thing is that it costs less than 5 bucks on the App Store—and yes I’d feel sore if I had paid $10 for it the week before, but really, even $15 for this thing is an absolute steal.