This is a question that's been posed in around 70% of the 100+ interviews I've done since being published.
It's a deceptive question that hides a lot of assumptions on both sides of the reading coin.
Many readers (interviewers) seem to think authors can reel off a list of names ... and perhaps some can, but then we're a diverse lot.
I've never liked the question for the simple reason that I don't have a good answer.
I often resort to citing the only clear and obvious influence, which is the influence for the character of Jorg.
Jorg's character is very definitely influenced by that of Alex DeLarge in Anthony Burgess's 1962 classic A Clockwork Orange. I've seen that spoken of as "Lawrence admits that..." There's no 'admits' about it. I've been telling this to anyone who asked since anyone started asking and it's been on the FAQ page of my website since it went up in 2010.
I've also seen people say "Prince of Thorns is a fantasy Clockwork Orange." This is certainly untrue. A Clockwork Orange is part satire, part social/political commentary about how society (in the 50s/60s) dealt with the excesses of teenage sub-culture (a very new invention at the time). Prince of Thorns is none of those things - and where Alex very deliberately has nothing in his past to muddy the waters about the origin of his behaviour, Jorg does have a troubled history.
What about fantasy influences?
When people are making these up on my behalf the one that arises over and over is Joe Abercrombie. I found out who Joe was in 2010 when on a French forum a publisher who read my manuscript said he loved it and it was the best thing he'd read since Abercrombie. I frowned, looked for the "& Fitch" then went to Google "Abercombie" and "fantasy". I've yet to read an Abercrombie book, and that's due in part to my slow reading rate (since Prince of Thorns was published I've read my first books by Rothfuss, Lynch, Brett, Weeks, and Bakker) and in part to what I will admit is a certain cussed resolve not to muddy the waters when I say he wasn't an influence.
Indeed if you look at the time-table of my writing you'll see I started writing Prince of Thorns in 2003 and by the time Abercrombie's debut came out in 2006 Prince of Thorns was 90% finished. Since I put each chapter out on my Yahoo writing group as I wrote it this is a matter of record. I belabour the point since I've become used to being point blank called a liar on this issue (this seems bizarre since I understand from many sources that our writing is not similar and Abercrombie doesn't write in first person), both from behind the anonymity of forum posts and (in one case) by a blogger. Even when there's no outright accusation I've grown used to seeing "Lawrence says he hasn't read", that subtle qualification 'says' introducing the element of doubt.
I've found that my refusal to 'accept' Abercrombie as an influence drives some forum dwellers to grow very hostile very swiftly. Don't care. He isn't. Nice guy though, lives 10 miles from me, met him a couple of times, had dinner. Very smooth. Handsome fella too.
So ... if not Abercromie, Morgan, Bakker, Rothfuss, Brett, Weeks, Lynch etc ... then who?
Well, here's the thing. I don't know. That's not how my mind works. Clearly the fantasy I've read influences how I write ... but it's not a conscious thing. I don't, when writing, think "I'll do this like XXXX." And afterwards, when I read it back, it doesn't strike me: "This is quite like YYYY."
It's certainly true that George RR Martin with his A Song of Ice and Fire series changed the way I thought about fantasy (which I had grown disenchanted with over the ten years prior to 2003 - hence the lack of the recent 'fundamentals' on my reading list). So, although my writing is nothing like GRRM's (multi-character points of view, sprawling, detailed description, love-affair with clothes, food, and architecture) I freely acknowledge him as an influence.
Beyond that I would point at:
Michael Moorcock: High levels of violence, no regard for genre boundaries, sprawling imagination confined within very short books, anti-heroes.
David Gemmell: Sense of the epic, focus on battle both personal and large scale, flawed heroes, fantasy with heart and emotion.
Stephen Donaldson: Epic fantasy set against a real world background, a very flawed and conflicted single point of view character, dark under-currents, a personal journey embodied in a 'global' struggle.
Stephen King: Strong characters. Set me thinking about _how_ he was having the effect on me that he achieved through the words on the page.
Terry Pratchett: Humour with heart. Just because a page makes you laugh doesn't mean it can't also bring a tear to your eye. Just because a line makes you laugh doesn't stop it being world-class prose.
Robin Hobb: I didn't read any Robin Hobb until after I'd written Prince of Thorns, but I had certainly read several of her books by the time I wrote King and Emperor of Thorns years later. Hobb's great strength is a depth and humanity in her characters. The rest of her writing arsenal is very well stocked too. I'm sure I unconsciously took something from how she dealt with Fitz's development.