I read cozy and historical mysteries, a bit of Paranormal/UF, and to mix it up, I read science and gardening books on occasion.
Eserine, Hemlock and Monkshood. Decisions, decisions. Definitely not monkshood. It seems the height of common sense to avoid any plant called the most poisonous plant in Europe. And a horrible way to go. Hemlock sounds painless and dignified if you believe Plato's description of Aristotle's death, but as Harkup points out, there are many different types of Hemlock; get the wrong one and it's a nasty, painful way to go. Even the "right" Hemlock might not be as painless as historical records would have us believe. All in all, too high a margin for error.
My personal "pick your poison" would have to be Eserine; while completely undignified, all anecdotal evidence points to a painless death. Victims have even been described as 'docile'. Even more in its favor, Eserine has a very high recovery rate if caught quickly, with no lingering effects. Sign me up for the "E" option should I ever find myself in a totally fictional world where I'm forced to pick my poison. Real life never looked so cheerful!
Themis-Athena points out in her update that the Hemlock chapter is rather more spoilerific than Harkup claims. I definitely don't refute that, but having read the Hemlock chapter with an eye towards this possibility, and having not read Five Little Pigs (yet), I will add this for anyone concerned about the possibility: I did not find anything in the chapter to be spoilery... BUT given what TA said, I suspect if I read Five Little Pigs in the very near future, there'd be enough in what little Harkup said and didn't say, to make the pieces slide together faster. Your mileage may vary, but I know that between now and my reading of Five Little Pigs I will have long forgotten the details.