Farmer, who died last week, might have been the first author I tried to collect. Not sure exactly because that was in high school some three decades ago but at that time used bookstores were full of stuff you just don't see any more. So while I don't completely remember precedence that was when I was also picking up anything I saw by Dick, Bloch, Moorcock, Ellison and some writers with smaller bibliographies such as Disch. Not sure why Farmer - it might have been the whiff of the risque that interested a high schooler (his three Essex House books weren't reissued until college), the somewhat metafictional conceits of his fake biographies, the big idea behind Riverworld, the variety of genres (perhaps subgenres is more accurate) or probably just the sheer volume and confusion of his output. This latter is always a key factor for a geek, mastering a mass of information that in some way doesn't really matter what the information is.
It's been probably 20 years since I've read anything of Farmer's but the Riverworld series is what's stuck most firmly. It had one of those "wow" setups and an elaborate story that wasn't quite like anything else. The final revelation of what actually caused Riverworld was something of a let down, mainly because after years and a few novels anything would have been a let down. Perhaps Farmer should have wrapped this all up in the first book and then used the rest as installments but SF&F writers tend to think big (or at least trilogies) when that's not always the way to go. But apart from that the Riverworld books had an abundance of ideas & situations that were constantly surprising.
Farmer's rethinking of fictional characters and then linking them (in what's come to be called the Wold Newton universe) seemed quite brilliant to me at the time though more pedestrian in retrospect, almost like slash fiction minus much of the slashing. To a large degree Farmer was breaking this ground. (I read Myer's Silverlock when it was reissued in the 70s--Wikipedia says 1982--but wasn't very familiar with its classic lit sources.) By now of course we have League of Extraordinary Gentlemen where Moore does something similar but for more serious ends.
Other than that not much of Farmer's work has stayed in my head if I even read more. I do remember A Barnstormer in Oz feeling like a distinct mistake and am pretty sure I got through Lord Tyger and at least a couple of story collections. Nothing from the World of Tiers series and probably little of the very earliest stuff. And of course his notorious sub-Joycean contribution to Dangerous Visions that really should have stayed in the desk drawer.