Most of the titles I collect are based on the character(s) -- Daredevil, Elektra, Green Arrow... Others are based on the artists or creators, but Buffy the Vampire Slayer is just a great, fun... thing. While I love The Slayer, it's not just her that makes Buffy so great -- Buffy the Vampire Slayer was its own thing, and it transcended both its movie and TV origins.
Many of the characters I follow are rather dark and gritty, so Buffy is a nice change of pace from the vengeful vigilantes and anti-heroes I usually collect, and has a range of toys, games, comics, and other stuff to collect. That's actually one of the reasons I've chosen to focus more on Buffy memorabilia. It has nothing to do with nostalgia, just a love for the show and characters, and something to do to pass the time.
Most recently, I picked up a copy of the Buffy the Vampire Slayer RPG Core Rulebook from Eden Studios. It's a copiously-illustrated hardback that looks pretty exciting. Collecting the OOP series will be expensive, but doable, and nothing I need immediately.
I also discovered a cache of TPB outside of the ongoing Dark Horse series for a fair price. In fact, most of the Buffy stuff I've come across is relatively affordable and available -- another reason I decided to invest more in it. I do not expect the majority of these items to appreciate significantly in value any time soon (or possibly ever, as most of them are common), but they'll make a nice "humblebrag" conversation piece, and fun to acquire.
I'm actually looking to upgrade a lot of my collections, so I may have some auctions to promote eventually.