Right now I think the best solution is no captcha. HN doesnt use one for the sign up form. Yes, the HN community is flooded, but for the most part I dont think a captcha would change that.
HN is a (largely) community moderated content aggregation site. What if you don't have the benefit of a community to moderate? If you're an email service and want to stop bots from signing up and spamming the world at large, how would you do that?