If you are wanting a stylus with true pen-like response, it does not yet exist.
The zoom feature in Penultimate is nice too, though the app is not nearly as feature rich as Notability. Depending on what you want to do that simplicity can be a good thing. It's good enough that a standard (fat) stylus creates decent writing.
Evernote sells the Adonit Jot Script; however it's not for everyone. I think it works fine with Penultimate, others consider it an across the board failure. The technology that allows a fine tipped stylus is far from perfected.
In my opinion it is entirely unsatisfactory for anything but writing. Slight variances in accuracy don't affect the legibility of writing, much, and can be compensated for. Drawing, however, is another beast.
There is at least one Kickstarter in progress for a stylus similar to the Jot Script. Delivery is likely to be months, if not a year or more away; and there will be no telling how effective the technology will be. Sorry, I don't have the link handy.