I tried to read this one because it's such a clever idea, but I couldn't finish because it does too good a job at imitating a second-rate pulp SF adventure built on a racist, fascist worldview. It eventually wore on me... For those who don't know, it's meant to be the book that Hitler would have written in his youth if he wrote SF instead of painting!
For me, hands down it's Moorcock's Elric saga. Writing fantasy epics that are hugely epic in scope as well as page count is one thing; doing it with the economy of word that MM used (in the 60's & 70's) is way more impressive. Many or most others will differ, but what I tend to dislike about epic novels is an excessive amount of detail given to the kingdoms, the settings, the backstories, etc. Just give me original, flesh & blood characters fighting in an epic struggle. In a few strokes, MM will give you all you need to know to enjoy the action.
A second pick and big influence on my ideas as a writer was Fritz Leiber's The Big Time for an intimate plot set against a huge cosmic backdrop.