Lets see, I think imagnative depends on era or decade as each had their big hitters.
I have been reading fantasy since the mid sixties, and it seems that each decade has its big hitters, but if I have to point to any work that got me hooked on fantasy, it would be Burroughs and Ms. Andre Norton.
In the seventies, John Norman's Gor series came close to Burroughs but with a much more adult feel to his work. He and his publisher even got death threats from the women libbers at the time, so much so, he stopped the series, or so it was reported at the time.
By the eighties there was Jerry Pournelle's Janissaries series, the Weiss and Hickmans's Dragonlance Series and of course, Donaldson's The Thomas Covenant Series, a LOTR like series with a more modern feel. And of course there was another grand dame, Ms Anne McCaffery and her Dragon's of Pern series.
By 2000, my reading taste had switched and I have not read a lot of fantasy lately, so I am unsure who the big hitters are today, though I have read a lot about a guy named Patterson.
As I said, if I have to narrow it down to what hooked me on fantasy, as in being very imagnative at the time I discovered fantasy, I think Burroughs and Ms Andre Norton hooked me.
Nick Anthony