Using Goodreads' wonderfully flexible shelving ability and covers collage - though with several modifications of my own for three titles that have no available covers there - I want to present the current list of my all time favorite books, ranked in order from 1-42, though from 13 on, the ranking is more fluid, especially the lower one goes on the list.

Titles sometimes mean series, occasionally just a sample of an author's overall work and I tried to keep to my rule of mentioning an author only once - there is one exception since the two series in cause are very different, while using covers from editions I own in as many cases as possible.

With 3 exceptions, all titles are available - though they are sometimes rare as my #1 title by Mircea Eliade, published as the Forbidden Forest rather than its original Midsummer Night - in English even if I included French or Romanian language covers which are more reflective and/or are from the editions I have. One of the covers below represents the longest book I own at 1620 pages though it's an omnibus of the seven novels of the series.

Edit: in the meantime I found a good short English description of my #1 title, The Forbidden Forest aka Noaptea de Sanziene HERE; note the "elements of fantasy" part, and while I would not really classify it as sff, I think it is quite associational in many ways and I strongly regret it is so hard to find in English outside good university libraries.

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1-12 and pretty firm ranking:

The title list is HERE.

Here there are six sff titles out of 12, with Iain Banks (#6), David Weber (#8), Peter Hamilton (#9), George RR Martin (#10), Jacqueline Carey (#11) and Adrian Tchaikovsky (#12). While one is a standalone though part of a loose series that stands at 8 novels and counting, the rest are part of series that stretch over many pages, from some 3000 minimum, with the longest one at 17 novels, 5 anthologies and going strong for another 10 maybe...

Considering that of the 6 non-sff titles, the only "slim" one is also part of a loose series of historical adventures - including having a direct prequel - with the rest ranging from 60o pages to 1000 and then to two seven volume series, my preferences for the epic are clear.

From the sff authors I have reviewed at least one title and for the non-sff books there is a little discussion in this post about recommended "non-sff for the sff-reader".

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#13-42 though the order is really fluid especially down below 24 or so

The title list is HERE

Here there are 13 sff titles and some strong associational ones like The Master and Margarita. There is KJ Parker (#21), Christopher Priest (#25), China Mieville (#27), Alastair Reynolds (#28), Jules Verne (#29), Eric Flint (#32), David Weber (again but with a very fantasy-nal series #33), Jack Vance (#34), Greg Egan (#35), Sean Russell (#36), Neal Asher (#38), Neal Stephenson (#39) and Lois Bujold (#42).

Same as above most books are part of series with few standalones and even those tend to be connected to other works of the author like say The Affirmation or The Scar.
For most sff authors above I have reviewed at least one of their works.