I made this post on the forums but wanted to preserve it as I get this question quite frequently.

My tastes lay strongly in Fantasy and Sci-Fi, but I'm what you'd call an Audiobook diehard nowadays - it's my favourite way to "read", and I gravitate to books on audiobook first and foremost. I barely use my Kindle anymore.

Anyway, here are my tips for the best fantasy and sci-fi audiobooks outside of Song of Ice and Fire, which I haven't read (and don't intend to unless Martin plans on finishing it any time soon - which was looking likely a couple of years ago, but now seems like a pipe dream).

ED: Note, the lists below represent them the the order I'd recommend them to others, not "my favourites" though my favourites are mentioned. I consider audiobooks just a great way of experiencing great books, so I would say that none of these take advantage of the format to the fullest (and they're certainly not radio plays), but rather are "just" incredibly enjoyable ways to spend your time.

Fantasy:

1. Mistborn - Brandon Sanderson is my favourite Author by far nowadays, and this was his first truly great work. I put it in poll position because not only is it engrossing, it is read by my favourite reader Michael Kramer and there are only 3 volumes with 1 spin-off, so it isn't too time consuming. Believe the reviews, it is really that good (averaging 4-5 stars everywhere).
2. Monster Hunter International - Consider this "light" or "pulp" fantasy, it's set in modern day and is the equivalent of a summer blockbuster movie. Constant action, fun characters, simple motivations, good humour, and a compelling plot. I put this in 2nd because it's the type of book I'd recommend to those who aren't that into fantasy knowing that pretty much anyone can enjoy it. Again, it averages 4-5 stars everywhere, and deserves it.
3. Storm Front (Dresden files book 1) - The first Dresden Files novel, and though not the best one, it is the tip of the iceberg for the rest of a fantastic series which defined urban fantasy and continues to do so. I put it here because it's only 8 hours long and if you do like it you're in for a lot of fun with the rest of the series. The plot is solid, and the reading is decent - and though this averages "only" around 4 stars, the rest of the series (currently 14 books or so) easily gets up to that 4-5 range.

Other notable, very good, fantasy audiobooks are the obvious, The Eye of the World (first book of the Wheel of Time) which is read by 2 readers including the afore mentioned Michael Kramer and my 2nd favourite reader, Kate Reading - or similarly you can check out the biggest new thing in Fantasy, The Way of Kings which is made by the same crew, is incredibly highly rated and deserves it. Codex Alera (6 books, each around 15 hours, and is a mix between popcorn and interesting fantasy) also read by Kate Reading is a solid series of well-read novels that are much under-appreciated. The Iron Druid Chronicles is another nice popcorn-y peice of fiction and The Blade Itself is fantastic if you like dark fantasy.

ED: Oh, and of course there's the KingKiller Chronicles which is another huge thing in the fantasy space at the moment - once again the rave reviews are largely well earned.

Sci-Fi:

1. Ender's Game - A seminal piece of sci-fi, everyone should experience it and the audiobooks are a great way to do so. Hopefully the ending hasn't been spoiled for you, but aside from that it's a no-brainer. Only around 12 hours and it's consumable as a stand-alone piece, so your commitment reqs are nice and low.
2. The Lost Fleet - The most fun I've had with sci-fi novels, it's not popcorn but somehow manages to be constantly engaging for the duration of the 6-book series. If you liked Battlestar Galactica, this shares many of its great elements (a long withdrawal with survivors trying to return home through the many dangers of space). Being scifi, each book is much shorter than a fantasy novel would be so the whole 6 books is (I believe) <70 hours of time, so shorter than any of my fantasy recommendations. Book 1 is 10 hours, is well read, and after finishing it I warn you of wanting to immediately purchase the rest.
3. 14 - A nice peice of "urban sci-fi" for those adverse to things set on space ships, this was a surprisingly enjoyable romp through many of the tropes of scifi with some legitimately interesting elements of mystery for the first 3/4. I warn you that the ending isn't the best, but it earns its positive reviews and is a well read 13 hours.

Other notable sci-fi has pretty much been mentioned by the other commenters here, there are so many solid sci fi classics that a casual browse through that category in Audible will yield a number of recommendations for me.