Identity Criss
I grew up on Marvel Comics, primarily X-Men. Never gave DC a look. Then in college, working at a comic shop, a co-worker started lending me various Marvel and DC runs they owned to broaden my awareness.
Enter : Identity Crisis. He knew I'd like a darker story, and this series delivered. I don't want to give away too many details, but the basic premise is that the Justice League has tiers of members. The Big 3, Batman, Wonder Woman, Superman. And then, everyone else. And sometimes, after a conflict, the others are left to clean up the mess as The Big 3 have to move on to bigger problems. This story explores it in way I very much enjoyed.
X-Cutioner's Song
Part of growing up on Marvel Comics started with the Animated Series of the 90s. To the point where it was the first time as a kid I really paid attention to what day it was and what time :D I'd make sure I'd be up in time to catch the weekly episode. My mother noticed this, and so grabbed a pre-bundled 12 issue story spanning multiple X-Men titles she saw at the grocery store she worked at.
I fell in love with it! I read and reread them over and over. Next we found a comic shop near home, and my odd job money had a focus that would continue for years!
I won't claim X-Cutioner's Song is the best story ever written. It is good, and has plenty to appreciate, but I understand my nostalgic bias. It is VERY 90s :D . It was just my on-ramp to paper comics, and it could easily be for others.
So if you're a long time reader, or just coming on board after watching the recent X-Men '97 series on D+, I can suggest this story as a great way to flesh out a wider cast of characters that the X-Titles showcase.
Age of Apocalypse
It doesn't happen every year, or even every 5 years, but it isn't uncommon for there to be Alternate Realities in Marvel Comics, or any established franchise in general. It's a way to reboot or explore in directions you never could as a writer in established canon.
Age of Apocalypse is my favorite Alternate Reality story. Taking 30+ years of canon, pulling threads from it, and turning the rest on it's head! Good guys are villains, or trapped by circumstance. Some characters are complete opposites of their established versions, but still have core threads of their character still being adhered to. That's the hook for me.
The set-up, what if Xavier never formed the X-Men? What if he'd died years ago? One man's worth? This series was pitched as a whole new start. We didn't know then it would only run 4 months. But it took over Marvel for those 4 months, and we thought we were never going back! I'm not sure something like that could be pulled off w/o spoilers in the Internet Age.
This was a bit of an end cap for me with comics as a kid. It was a tough act to follow. Other creators came on board, the industry changed, I had gotten older and developed other interests. Life. But this is a pinnacle story telling in my opinion.
I didn't originally plan for all the of my suggestions to be comics, but here we are :D