How do you use Instapaper now, toronado?
My definition of "best" in this case means automating the process, your definition may be different. You might want the eReader with the best browser so you can access the Instapaper webpage or you might even be asking about tablets rather than eink readers, in which case the iPad would be best since Instapaper is an iPhone/iPad app. The developer of Instapaper uses a Kindle so he's optimized the process for Kindle, even organizing the Instapaper downloads like magazine articles that automatically archive so you don't have to do any organizing yourself.
If anyone reading this thread is interested in setting up Instapaper for their Kindle,
this blog post walks you through the set up process and shows you how it works. It really is a neat trick and I use it all the time to save long blog posts and articles to my Kindle so I can read them later.