Audible are the best out there imo. Selection is pretty extensive, And the 12 month return option can be very useful.
I'm currently on the 2 credit per month subscription. If I need more credits than that, they're £18 for 3 iirc. Works out pretty cheap.
Library for me totals 863 books/radio series right now. Also recently hit 15 months of total listening time through the app... Ouch...
Currently listening to Bobiverse Trilogy, really enjoying them. But Bill Bryson, Terry Pratchett, Tom Clancy, Yahtzee Crowshaw, Tony Hawks, the Alien series of books and so on are all very good. But I've also got a lot of radio series on it. I'm Sorry I Haven't a Clue Treasuries, Dad's Army Radio Show, I've Never Seen Star Wars, The Museum of Curiosity, Hut 33, Star Wars Radio Show, Infinite Monkey Cage, Just a Minute Collections, Yes Minister...
The daily deals they do are sub £3 and if they've got what you want, they're a very good deal. They recently did a daily deal revisited thing, they brought back 100 former daily deals for that price each day for a week or so. Then there's the various 3 for 2 and 2 for 1 sales that happen quite often. Only around 300 books in each one, but I can usually find a couple that interest me. Then there's the cheap audio books for some Kindle books, so you can get both for a reduced price. Called Audible Matchmaker iirc.
Let's add to that the fact that if you have an active subscription, you get access to the library of Audible Original Podcasts for free. Many of which are fantastic. They call them podcasts, but they're not. It's normally a series of 30 minute shows. Some live comedy, some drama, some educational, some radio shows (the complete Unbelieveable Truth is on there as is Fry's English Delight...) and other stuff.