Assuming you are buying a NEW iPad, the iPad Pro models (9.7" and 12.9") support the Apple Pencil (only the Pro models though - the Air 2 and Mini 4, as well as all other iPads, do NOT support the Apple Pencil). This is the BEST (without question) experience for drawing on an iPad.
There are stylus that work on ANY iPad - the capacitive stylus. These are the ones with the rubber tip (usually round). The best (but not cheapest) version of this stylus is the Pencil by 53.
Additionally, there are fine point Bluetooth stylus out there that work with "most" iPad models (they don't work well with some older models). The point on these stylus are very small, but all of them suffer from problems with lag and parallax (I.e. The mark you make is off from where you intended it to be). There are many options, but the ones from Adonit or Wacom are probably your best bet.
There are many stylus out there that are absolutely NOT compatible with iPad. These include stylus that are made for Windows and Android devices. Often these stylus are only compatible with specific models and not even compatible with other devices using the same technology (example: The Surface Pro 4 pen is only compatible with Surface Pro 3/4 devices, but not Pro 1/2 devices).
The Bamboo Fineline is probably the best in it's class (non-Apple Pencil, Fine point, Bluetooth). I haven't tried the Fineline 2 or the newer Wacom Omni, but given how good the Fineline is i'D imagine it's pretty good. I was disappointed in Wacom's Creative 2 stylus though.