For me it was 2000AD 690. Picked it up on a whim as a 15 year old on begrudging holiday with my family. It was bang in the middle of the Necropolis story, and everything about it seemed fascinating to me...
It caused me to collect both forwards and backwards, desperate to know what had happened before (which was a lot, since this storyline had been brewing for years) and what happened after.
It was also a gateway drug into US comics, primarily Marvel, but pretty much anything that looked cool at the time. I still have all my physical books, stored in a bunch of long boxes up in the attic that I swear I'll get down and look at one day, but never do...
Hellboy. I love the setting of having all these fairy tale and folklore characters existing just under the surface of the world and the way Mike Mignola draws is stunning. I have spent so much money on Hellboy comics and I really wish I still had the income I had a few years ago to buy more!
I also bought quite a few of the Judge Dredd collections myself, it's a great comic series
I remember having some comics as a kid, nothing huge or noteworthy. Eventually started buying ye ol Archie Comics in the checkout aisle, reading comic strips in news papers, not much more. Eventually as years passed (and as a huge Marvel fan but not having actually read any Marvel comics, mostly just tv like Spider-Man/X-Men TAS) I heard of a storyline in the comics which was just starting which intrigued the hell out of me called House of M...and from there things just never stopped!
I started collecting comics from the spin rack at the grocery store in the 90s. I was mostly reading Spider-Man and X-Men. I got back into comics in college since I lived close to a comic store. The Bendis/Maleev Daredevil run and the Loeb/Lee Hish run got me back into comics for a few years. I got back into comics years later with Saga and I am still collecting.