There is no single, absolute answer to this question.
There are many factors that have to be taken into account in order to answer it.
The simplest answer is this: "The best tablet for graphic design is the one that's best FOR YOU AND YOUR CIRCUMSTANCES."
Factors that might count...
1. Budget.
Do you have a lot of money to spend? Buy the most expensive machine you can afford. This might be an iPad. And it might be a 27" Wacom Cintiq (with a highly pumped up gaming computer to drive your software as fast as possible).
I have a simple rule for myself when buying technology... If I can cover the purchase price in one or two invoices to my clients, then I can afford it. It's a simple matter of generating the extra work.
2. Portability vs Fixed Location.
I use a Wacom Cintiq Companion 2 at the moment. I used to use a Fujitsu T901 tablet pc. Before that, an Asus tablet pc. And before that, a Toshiba Tecra M4 tablet pc.
In my line of work -- visual facilitation, where I go to conferences and client powwows, and draw what they're saying -- my tablet, and everything else that goes along with it, has to be able to fit into a bag that I can take with me into any airplane as hand luggage.
You might have a different set of needs. If you need portability, you'll find that the best tablet is a portable one. If you're stuck at a desk, a giant tablet would suit you best.
3. The software you want to run on your machine.
I use Photoshop CC2015 for my drawings, and Premiere Pro for video production. For my needs, the top-of-the-range Companion 2 works perfectly.
However, I know people who use BRUSHES on their iPad to sketchnote meetings.
If you need high-end software, you'll also need high-end hardware.
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In conclusion, work out what factors are important to you. Then choose the most expensive, most highly specced machine you can afford.
