We've worked together enough to know that my answer is... it depends!  My first recommendation is always https://www.authorsrepublic.com/ because they will distribute to all the different outlets that you'd like (including Amazon) with more control over the process and access to the library market.  However, for strict ease of use, Amazon is still tops - especially if you've got your print/ebook already available through their site.  One thing that several of my authors are doing is starting out with Amazon on a 90-day exclusive basis (because you maximize your Amazon royalty that way and have access to their promotional tools like giveaways etc.) and then switching to non-exclusive after 90 days. So that's an option.