Except where noted, at last check all of the below is on Marvel Unlimited
If you haven't read it already, in the 80s I'd start with the original Wolverine four issue mini series from 1982, written by Chris Claremont, art by Frank Miller. That's the one they messed up in the most recent Wolverine movie. Then there's the 1st Wolverine ongoing series that started in 1988. Similarly dark and violent are the original 5 issue Punisher miniseries (1986) and the 1st Punisher ongoing series starting in 1987 and Punisher: War Journal (1988).
In the X-men, there is arguably the most famous X-men storyline, the Dark Phoenix saga, from 1980 (The X-Men #129-138). Then the original Days of Future Past from 1981 (Uncanny X-men #141-142). And the miniseries Kitty Pride and Wolverine (1984) is also good.
The next obvious one is the original Secret Wars (1984). They are about to do another Secret Wars this summer (I am sure the original is better). And there is a Deadpool special coming soon, with him inserted into the original Secret Wars (he wasn't in it as he wasn't created until 1991).
From there, the Spider-man from right after Secret Wars is the obvious place to go. Spider-man gets his black costume in Secret Wars, and only afterwards discovers it is an alien (Venom). So the original Venom stories are from then. And then the Todd McFarlane Spider-man from 1988 (which includes more Venom) arguably the best Spider-man art since Ditko.
And then there is the best Thor run IMO - the Beta Ray Bill saga, writing and art by Walt Simonson, starting in Thor #337. The other Walt Simonson Thor is also excellent, but Beta Ray Bill is the best Thor from that period. (I just reread the Beta Ray Bill saga last week).
And then there is the stuff from the 90s (some of my personal favorites). The first group of Thanos Infinity comics- Infinity Gauntlet (1991), Infinity War (1992), Warlock and the Infinity Watch (1992), Infinity Crusade (1993). The Joe Kelly Deadpool is also from the 90s, but that is not in Marvel Unlimited (they make a lot of money selling that in a hard copy omnibus, which is worth owning).
And to sound like a hard-core comic book dude, you also have to read a bunch of the Brian Michael Bendis from the last decade that is about to dominate the movies and Netflix - all of the coming Marvel Netflix is derived from the Bendis runs on those characters (one of whom, Jessica Jones, he created). And he wrote more Civil War tie-in issues than anyone else. And about 3 times as many issues of Daredevil than Frank Miller did. Plus his run on Guardians of the Galaxy.
And in more recent and comical, the Daniel Way Deadpool (2008), Deadpool: Merc with a Mouth (2009), Deadpool Corps (2010), and the Brian Posehn/Gerry Duggan Deadpool (2012).