On the topic, I'd say my default would be to say Lord of the Rings/Middle-Earth is obvious.
Others have already mentioned it repeatedly.
So...I started to think if there could be a greater Fantasy franchise which has had a greater impact on history, writing, and works than Tolkien's works?
Then it struck me...yes...there are actually a few. Then I also thought, if I mention them as a fantasy or science fiction or anything like that...it will not be seen positively and people would lynch me.
So...then I thought of how to put one of these in a way where most would not be offended...and maybe even laugh??
So...in trying to top the Lord of the Rings on relavance and impact...I'd say...Greek Mythology.
It has to be the single greatest fantasy (and it was a group effort) ever created and has had the greatest impact of all various types of fantasy on WESTERN culture (not as great an impact on other cultures). Modified and utilized by the Romans, that counts as a variant of the Greek Mythology still in my mind and opinion (though one could argue the Egyptian mythos is either a precursor, and adaptation, or a separate branch from the same tree as well) and all their impacts that they've had.
Norse Mythology also has had an impact, but I'd say Greek Mythology solidly stands above it.