Again, I reserve the right to update this list as I think of new books tp include (I'd have 2800+ novels in my personal library so I can't think of every one offhand).
Paul Kearney (Monarchies of God, Sea Beggars, Macht)
One of the series has never been completed due to lack of a publisher, yet his books are all good and the *Monarchies of God series (1400s not-Islamic invasions with werewolves etc) is really interesting and has some great battle scenes. Recommended.
James Enge (Blood of Ambrose)
This story of the life of Merlin's family (the Ambrosii) has an uncommon writing style - it reminds me of the fantasy books I read in the 70s, but with a modern voice. Like hearing your grandad use modern slang. It's catchy, interesting, and unique in its style and pacing.
*Traitor's Blade (Sebastian de Castell)
This "Three Musketeers with Magic" is a swashbuckling romp that won't be underrated for long. A new author who grabbed my attention instantly. This is a good book, a fun book. Highly recommended. One of the unexpected hits of the year.
*City of Stairs (Robert Jackson Bennett)
Another newcomer who grabbed my attention. A highly original fantasy where a once-mighty city has lost its gods and is now just an outpost of its old colony. Some memorable characters and a gripping plot premise. Unusual in style. A great mix of genres from spy thriller to fantasy. An author who deserves a wider audience.
Anything by Chris Wooding
Speaking of fun, Retribution Falls* is not strictly fantasy (more steampunk) but it is an awesome mashup of golems-airships-sky pirates-dogfights-demonologists. Pure fun. A steampunk Firefly if you will. Whilst more workmanlike than genius, most of Wooding's other work is solid to good, such as the Weavers of Saramyr Asian fantasy series.
Shadows of the Apt (Tchaikovsky)
He's only a workmanlike author but his world building is good. His fantasy human races have insect traits (spider kinden are schemers, ant kinden are telepaths, beetle kinden are industrious) and whilst it doesn't perhaps need to be dragged into a 10 book series like it has, it's a decent read.
Chronicles of Prydian (Lloyd Alexander)
A kids book, not really "unknown" to its target audience, but I found the French-Revolution with magic fantasy much better than Narnia. Not a bad "light read" as an adult.
Anything by Guy Gavriel Kay
He's not exactly unknown, but underrated, sure. Since his writing skill edges nearly all the authors on the "top 10" lists, I think he deserves to be in this category, and his work is definitely worth checking out. He often tends more towards historical fiction than fantasy.
Tales of the Kin - Doug Hulick
Another one of those gritty "fantasy crime" books which are flooding the market at the moment. When the Lies of Locke Lamora gets all the plaudits, I feel this one is underrated in comparison. So that's why it's here.
*The Witcher (Sapkowski)
Certainly not underrated if you are Eastern European, where he is "the" GRR Martin/Tolkien. But whenever I say "Witcher" all people know of is the PC game that derives from this series. It suffers from some rather awful translation work, but is still good. I recommend the short collection of twisted gritty fairytales "The Last Wish"as a starting point.
The Darkness that Comes Before (Scott Bakker)
Sometimes more philosophical discourse than fantasy book, it overreaches itself at times. A bit too much navel gazing, but unique and interesting. A different flavour of fantasy for the jaded palate, though I recommend it with caution - it's not for everyone.
Low Town/*Those Above (Daniel Polanksy)
The Low Town series (about an ex-Imperial agent turned crime lord/drug dealer) was a bit grim and dank to wholeheartedly recommend. A solid gritty-crime-fantasy though. I did re-read it though. But his new world introduced in the series which leads with the book Those Above looks to be great, and I predict he will climb into the "top 10" lists when it gets more traction.
Fallen Blade - Jon Courtney Grimwood
Mr Grimwood has always written good but unusual sci fi but he is a talented writer and I'm surprised his werewolves-vs-vampire assassins in Venice hasn't got more notice, given how it taps into a popular theme.
Killer of Men* (Christian Cameron)
Technically historical fiction but with gritty fantasy nowdays the line is blurred. It's weird as his other work is laboured and poor in style, while the Killer of Men series are my favourite Greek historical novels.
Dagger & the Coin (Abraham) + Long Price Quartet series
Daniel Abraham is one of the American midwest clique and is probably the most talented writer. His flaw - he's a bit slow to build the action. That said, he's far less famous than the others who are inferior writers, so I'd definitely tag him as underrated.
Brian Ruckley (The Free)
First books a bit derivative and long winded (I liked the American Indian Elves vs Vikings battles from the Godless World), but standalone The Free (mercenary company with magicians) will be appreciated especially by wargamers for its platoon-level gritty battles and balanced magic.
Anything by KJ Parker
I'll nominate the standalone "Sharps"but all are similar in tone. They are well written in a wry way, and are a lot better written than their modest fame suggests - perhaps because KJ Parker can be a bit grim in large doses (industrialisation, money and banality always triumph over gallantry and flair).
JV Jones (Sword of Shadows)
I remember being impressed with this series but they were published only sporadically (the series has yet to be completed with often 4-5 year gaps between books). Quite grim and gritty, but different and good compared to the "standard" gritty which has become the norm in fantasy.
Django Wexler (Thousand Names)
The first book started poorly but improved as it went. The second shows solid improvement over the first. I'm now actually really looking forward to book three. It's basically Napoleon-in-Egypt hunting demons, then a fantasy French Revolution. A series to watch, which should definitely find a wider audience.
Underrated, not epic
Well there you go. Not necessarily the best fantasy you will ever read, but all decent, enjoyable books with authors which tend to slip under the radar. I've resisted the urge to include a lot of newer fantasy authors (and there are a lot of those: fantasy is at the moment, at the cutting edge of literature compared to the tired crime/thriller genres, and sci fi which had its golden age decades ago.) However the series tagged with a (*) are ones which I'd say deserve to be in everyone's library.
I hope you find something interesting and new in this list - if you want more detail, or you have a book/author that deserves to be here, toss it in the comments below.