Man, after putting out three audiobooks myself, I've gone down this rabbit hole. ACX is the giant, obviously, and for a reason—it's the main pipeline to Audible and the Apple Books storefront. The royalty-share option can feel like a lifeline if you're bootstrapping, matching you with a narrator who works on spec. But it locks you in for seven years, which is brutal if your book takes off. I tried that for my first and regretted it.
For more control and a wider release, I switched to using Findaway Voices for the next one. They distribute to something like forty platforms, including libraries, which ACX doesn't touch. The dashboard is cleaner, and you can set your own retail price, which is huge. The trade-off is you're footing the production bill upfront. I found a narrator I loved on Voices.com, paid the fee, and then used Findaway for distribution. It hurt the wallet initially, but the long-term split is better.
Honestly, the "top" service depends entirely on your goals and budget. If you want the Audible audience at all costs and have zero capital, ACX royalty-share is your only real starting point. If you have some savings and want to own your files and reach more listeners, the combo of a casting site plus a distributor like Findaway or Author's Republic gives you way more flexibility. Library access through Hoopla and OverDrive has been a surprisingly steady source of income for me.