Wacom's own tablet stylus is, according to most, the best you can buy for tablets. $30. [link] If you want something precise, there's the "Adonit Jot Pro" which has a weird disc-pen thing on the tip. It's the same price as the wacom one, but it's fragile and apparently has conductivity issues. [link]
With the advent of Windows RT and 8, a bunch of tablets with pressure-sensitive screens are starting to pop up. For example, the Asus Vivo Tab has an official Wacom digitizer and pen combo built into it, with pressure sensitivity and everything.
You know, if you took the time to properly look at those links, it would/should send you to a conceptart.org thread regarding the Jot touch pressure sensitive stylus, along with comparison and discussion regarding pressure sensitive stylus' for the iPad.
and the other link to the jaja website to purchase the jaja pressure sensitive stylus.
It is the best stylus for the iPad for illustration IMO, why? because it offers pressure sensitivity!