These days, archival collections of The Defenders are likely to lead off with the "Titans Three" story (from Sub-Mariner #34-35) or even the "Undying Ones" crossover trilogy (from Doctor Strange #183, Sub-Mariner #22 and Hulk #126), but I like to go back further still. No, I don't go all the way back to Avengers #3 (although I could), I do go back as far as Tales to Astonish #84 (which I read for the first time reprinted in Marvel Super-Heroes #39. With the help of Mike's Amazing World of Comics, I have been able to determine that Marvel Super-Heroes #39 was probably my fifth Hulk comic (behind Hulk #167, Marvel Super-Heroes #38, Marvel Feature #11 and Hulk #168).
Tales to Astonish was a "split book" and #84 is a "crossover" in the sense that Sub-Mariner "crosses over" to make a guest appearance in the Hulk's story. Although the two don't even meet face-to-face, the Hulk at least catches a glimpse of Subby across a crowded movie theater. The Sub-Mariner is suffering amnesia (due to the effects of an explosion in the previous issue) and is being manipulated by the mysterious, red-robed "Number One" of the Secret Empire. Number One wants Namor to destroy the Hulk for reasons of his own, and set Namor loose in Manhattan to track him down.
But the story really begins (AFAIAC) in the Hulk feature, which is the start of a new story arc. It opens with the Hulk raging in the southwestern desert. "And then, some dim, forgotten memory returns to his clouded brain, as the green-skinned giant vaguely recalls a time months ago, when he was briefly part of a team--when he had powerful friends to turn to."
"AVENGERS!" shouts the Hulk. "Avengers will help! Hulk must go to them!" Understand, this is the very first time I had ever heard of the Avengers. I had no idea what to expect. "Avengers" was just a name to me at the time. "The once-brilliant brain of atomic scientist Bruce Banner--having been affected by the mysterious gamma rays, some years ago, can now barely remember that the headquarters of the mighty Avengers is located somewhere to the east -- and so. without a second's hesitation, the incredible leg muscles of the mightiest mortal to walk the Earth, begin to hurl him skyward--on a tireless journey half-way across a continent."
Three quarters, more like. I don't know how long that trip took him, but it's impressive he managed to hold the idea of "Avengers" in his head the entire time. He lands in the pre-dawn darkness, breaks into a men's over-size clothing store and steals an overcoat and hat. Later that day, the Sub-Mariner spots a movie theater showing newsreel footage of the Hulk and steps inside, hoping to find a clue to his whereabouts.
"There he is... on the screen before me," thinks Subby. "I have faced him before! I know it! But... I cannot remember where... or when!" A footnote informs us "If Namor were in full possession of his memory, he would surely recall having battled the incredible Hulk while the green-skinned rampager was a member of the mighty Avengers!" There's that name again! The Sub-Mariner is wearing a microphone around his neck so Number One can communicate with him, which brings him to the attention of the other patrons, which causing a ruckus.
Meanwhile, outside in the light of the day, still thinking about finding the Avengers, the Hulk is spotted by a beat cop who sounds the alarm. Now on the run, Hulk ducks into the same movie theater Sub-Mariner entered minutes before. Hulk enters just as Subby is being chased from the theater. "That face!" Hulk realizes. "I know him! I have seen him--somewhere--fought him--somewhere! But--I have fought so many--how can I be sure?!!"
And that's basically it for this encounter. I have put the events in roughly chronological order above, but each half of the comic is told strictly from each respective character's point of view. Two panels of Gene Colan's Subby story are even clipped and brought over to the Hulk's. The Hulk finishes watching the newsreel and leaves the theater, but a gust of wind blows his hat off. He is recognized, flees to the subway, accidentally damages the tracks, then saves the train. By the time he's done all that and bursts through the street, he has forgotten why he came to New York in the first place.
"Nothing stops the Hulk!!"he declares. "I'm free! Free--to do--what??!"