Ring of Fire, with abysmal results. It was really just a test. With the first giveaway, I spent a large chunk of time pre-promoting it and even spent a not-insignificant amount of money getting it listed on other sites. Downloads were upwards of 6K, but ensuing reviews were nil and sales didn't even cover the cost of the promotion.
Second giveaway was nearly identical to first, except I spent a little less money, the same amount of time, and got slightly fewer downloads (5K+). I hit #3 in Suspense Thriller, for Pete's sake, so I definitely got exposure. Again, sales and reviews afterward: nil to nearly nil.
SO, I thought for my last giveaway, let's just pay ONE person on Fiverr.com to blast it out to 25 sites (same resource I used for first two giveaways), do NO promotion myself and see what happens. Here's what happened: fewer than 500 downloads, and of course no follow-up sales or reviews.
So, promotion helps, if you're looking at sheer downloads. But do downloads translate to sales or reviews? Not in the least, and I have two books out there, so there's even another book to benefit from all this. I did NOT use some of the more expensive sites that I've heard do produce better results (Bookbub, for example), but then you're looking at spending $300-500 or more. I can't rationalize it.
In the end, I'm done with KDP Select. It was good to try it, but in the past year, Amazon has gotten beyond saturated with indie books and at this point, getting one book noticed through giving it away is like the Whos trying to get Horton's attention in that enormous field of clover. To grant them exclusivity for three months and get so little in return--no thanks.
For now, it's back to writing. That's a lot more fun, anyway.