I'll go for three characters from both companies for now:
DC:
- Superman: All-Star Superman, Superman Smashes the Klan, Grant Morrison's Action Comics run, Birthright. I also personally love a lot of the 70s and early 80s stuff, but it's not exactly idealistic introductory material and for obvious reasons is very much out of date now, so I can't recommend it as a starting point.
- Batman: Paul Dini's Detective Comics run, Gotham City Sirens, Batman: Year One, The Killing Joke. Let's be real for a second; if you're considering getting into comics then you almost certainly already have a strong grasp of the character and his cast thanks to his excessive push in other media. As such, I focused less on what would be a good introduction to the character and more on what would be the easiest for people who know him from stuff like the movies, the DCAU and the Arkham games to transition into the realm of comics, and you'd be hard-pressed to find something better-suited for that than Paul Dini's writings. Year One and TKJ are mostly there due to their iconography, but they're also still useful.
- Wonder Woman: Anything Marston did with her before he died, Greg Rucka's Rebirth run and George Perez's Post-Crisis revamp. Head straight to the Gail Simone, Phil Jimenez and Post-Crisis Greg Rucka runs after that. There was also a one-shot story by Steve Orlando involving Diana redeeming a C-List villain of hers that I absolutely loved and would highly recommend, but it's not a priority either.
Marvel:
- Spider-Man: The Lee/Ditko & Romita eras, Kraven's Last Hunt, Spider-Man #400: So the classic stuff is an obvious must-grab and KLH is commonly hailed as the greatest Spider-Man story ever told, but why Spider-Man #400? Well it's simple; I think it's one of the most beautiful send-offs to a character in all of comics, and in my opinion every Spider-Man fan should read it at least once.
- Thor: The Lee/Kirby material, Walt Simonson's run: As with almost every property from Marvel the original Thor stories by Lee & Kirby are basically mandatory to fully understand all the other shit that happens in the future, plus they're just fun. Walt Simonson is almost universally recognised as THE Thor writer though, and his work is some of the most epic material you'll see in a comic.
- Hulk: The Incredible Hulk 1-6, Peter David's run, The Immortal Hulk. So once again it's best to start at the beginning with The Hulk, though I will say that for the most part it's not nearly as essential as Thor or Spidey's initial stories, Peter David's Hulk redefined the character after a couple of decades of him being mostly static and The Immortal Hulk, while not exactly the most conventional Hulk series out there, is a fantastic return to the character's roots in a way that doesn't feel disrespectful to anything that David or any of the other Hulk scribes did with the character in the decaades between them.
Now obviously this is just my take on it; others will say completely different things for each character (Secret Origins or Byrne's Man of Steel for Superman, TDKR for Batman, Slott's run or Life Story for Spider-Man, Jason Aaron's Thor and so on), and some of those I do actually like quite a lot, but to me this is the best way to start with these specific characters.