First Splatbook you begged your DM to approve.
Should we explain this one? In the world of today, splatbooks would be equivalent to DLC for a video game. They were slimmer books that had laminated or vinyl covers–hence the “splat” when you threw them on a table. The oldschool 2e ones had these gorgeous faux-leather situations going on that captivated me as a young man. I remember parking myself in the stacks at the Borders…B. Dalton? I think B. Dalton in the Casper mall and reading through the Psionics Handbook for the first time in. Incredible stuff.
Again, though, I was usually the DM so begging came -to- me, not from me. When I first started gaming with the guys I’d play with through most of high school, I rolled a Firbolg. I don’t think I needed to convince them of this, though; I don’t even think I owned the Humanoid Handbook at the time, so they probably put it in my hands. I also believe that party included a dinosaur-man and a flind, so we weren’t really a Fellowship-style group.
Online-gaming-wise, I tended to find a solid group of dudes and primarily work games with them, meaning the accepted materials were usually pretty set. Back in the days of the Pits of Evil, most of what we used was homebrewed, because that was much more of a thing in 3.x than 4th. I think it’s helped me as a player that I’m never a powergamer and rarely predictable, so if I want access to a new book it’s so I can make a paraplegic centaur relying on an intricate latticework of unseen servants to cart his otherwise immobile horsehalf around the dungeon, while he lays about himself with a flail made from the plaited manes of the three sisters he watched slaughtered as a child. Someone on the other side of the table is probably building something worth considerably more concern.