Answer: The ones that have a resistive screens and the ones that have capacitive screens. There is stylii support for both.

I personally prefer an active digitizer that can registers only the pen tip (and pressure and orientation), as I like to rest my hands on large writing objects rather than hovering over them. So far, no 'tablet' has this killer feature, though I recall a software library by 'Ten One Design' (think: pogo) for iOS that apparently could distinguish between the two. Here's hoping we see more solutions like this in absence of the real-deal.

Here's a really stylus for capacitive screens, and hey, it includes free shipping!
http://www.dealextreme.com/details.dx/sku.25006