The whole 'best service' debate really depends on what you're trying to accomplish. If you want the widest distribution and print-on-demand setup without upfront costs, Amazon's KDP Print is unavoidable. Their integration with the Kindle store is a huge advantage. But I've had issues with print quality consistency on glossy covers – sometimes they come out perfectly, other times they look a bit pixelated.
For a project where physical feel mattered more, I used IngramSpark. Their paper stock options are better, and getting into brick-and-mortar stores is a real possibility, which KDP doesn't offer as effectively. The downside is the setup fee per title and the more complex dashboard. You need to be meticulous with your files.
Some authors I know swear by mixing both – KDP for the primary Amazon channel and IngramSpark for expanded distribution. It's a bit more work managing two listings, but it maximizes your reach. Just watch out for the 'double-whammy' of returns if you're not careful with your pricing.
Honestly, after trying a few, I think the 'best' service is the one whose quirks you learn to navigate. The perfect, hassle-free platform doesn't exist.