Iain M. Banks.
Originally I picked up The Algebraist off the “Recommended Sci-Fi” shelf at a Borders bookstore. Couldn’t get into it. Too dry. Let it sit for a couple years. Came back to it to decide whether to sell it or not, started reading it again, and finally got hooked.
After that, I inhaled all of the extant Culture series books and the few other non-Culture sci-fi books he had written. And read all his output right up through the final Culture novel.
As a kid I collected the Robotech books. Maybe not my first collected set but certainly the longest.