This list is restricted to "epicish" fantasy, which by my off the cuff definition generally involves a secondary world and warfare based on sword and spear.
The reason for such restriction is simple - it's my favourite kind of fantasy, and were I to include YA fantasy, urban fantasy, paranormal fantasy, romance fantasy etc the great majority of the the books below would be swept off the table because the truth is that "epicish" fantasy is a niche genre and sells far fewer books than the kinds of fiction I just listed.
I crowd sourced this list over at r/fantasy and if you have a debut book you think fits, please mention it in the comments.
The numbers in brackets are the number of Goodreads ratings the book has. Since this is directly related to sales it is a clear indication of the "size" of that debut.
I have included some titles that aren't really "epicish" but don't seem to fall into the other categories I mentioned either. They are probably close to magical realism, which crosses over well into the general readership and gets big sales. I've left them (shaded red) as reminders that the kind of fantasy shown here constitute small fish in a big pond!
Where an author has debuted in epic fantasy but had previous books I've shaded them grey. Marlon James, for example, won the Man Booker Prize with an earlier novel. So his debut in epic fantasy is not his debut.
Where an author has been published under a different name before I shade their debut under a new name in blue.
Where I had more than three I have stuck to the top three except in cases of ties.
2007 - The Name of the Wind (596K), The Summoner (11K)
2008 - The Way of Shadows (134K), The Warded Man (96K), The Steel Remains (14K)
2009 - The Dwarves (9K), Lamentation (3K), The Adamantine Palace (2K)
2010 - The Hundred Thousand Kingdoms (43K)
2011 - The Night Circus (572K), Prince of Thorns (80K), Blood Song (62K), Theft of Swords (47K)
2012 - The Red Knight (13K), Malice (12K), Throne of the Crescent Moon (10K)
2013 - The Golem and the Jinni (86K), Promise of Blood (34K), The Thousand Names (15K), Senlin Ascends (10K), House of Blades (10K)
2014 - The Emperor's Blades (30K), The Goblin Emperor (19K), The Shadow of What Was Lost (17K), Traitor's Blade (14K)
2015 - The Library at Mount Char (25K), Dawn of Wonder (19K), Free the Darkness (12K), The Grace of Kings (10K), The Traitor Baru Cormorant (10K)
2016 - Benjamin Ashwood (3K), The Crimson Queen (3K), Hope and Red (2K), The Summer Dragon (2K)
2017 - The Bear and the Nightingale (74K), Kings of the Wyld (15K), Jade City (4K), Blackwing (3K), The Rage of Dragons (3K)
2018 - Children of Blood and Bone (84K), The Poppy War (18K)
2019 - Candidates include: Black Leopard Red Wolf (6K), The Raven Tower (4K), The Ruin of Kings (2k), The Gutter Prayer (1K), The Ten Thousand Doors of January (1K), Gideon the Ninth (0K), The Black Hawks (0K), Master of Sorrows (0K), Shadows of the Short Days (0K), Blood of an Exile (0K).
Red = not 'adult' &/or 'epicish'.
Blue = previously published under different name.
Grey = previously published in different genre/sub genre.
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