The Hulk is Incredible, Indestructible, Rampaging, the Strongest one there is, and a noted smashing enthusiastic. What he is not, however, is undefeated. Before his current insane level of strength, where he can basically punch other dimensions, a variety of heroes and even a villain or two have managed to beat the jade giant into submission and/or unconsciousness…
1) Iron Man
Hulk's eventual science bro managed to lay the Hulk out with a single punch in Iron Man #132. Admittedly, it was a super-punch that used all of his armor's energy, rendering Iron Man powerless and still nearly killing him, but a KO is a KO.
2) Silver Surfer
If there's one being in the Marvel universe who can be relied upon to TKO the Hulk whenever he wants, it's the Silver Surfer. In Tales to Astonish #92-93, after literally smacking the Hulk with his surfboard and knocking the Strongest There Is on his ass, when Hulk got annoying later, the Surfer just rendered him unconscious with the power cosmic. In The Incredible Hulk #250, the Surfer wanted to break through the Galactus-made force field keeping him on Earth, and asked the Hulk if he could help. The Hulk flips out, and an annoyed Surfer literally just removes the gamma radiation from Hulk's body with the power cosmic, curing Bruce Banner.
Wolverine vs. the Hulk is one of Marvel's most popular match-ups, but outside of What if? And alternate universes, did you know Wolverine has never truly defeated the Hulk? Well, that is except once, when he was acting as the mutant Apocalypse's Horseman, Death, and was ludicrously overpowered as a result. He trounced the Hulk, and could have easily beheaded his foe until a stab of conscience changed his mind.
In a 1981 special, Batman took out the Hulk is an absurdly efficient Batman kind of way. After a mess involving the Joker, the Shaper-of-Worlds, and a fake general, the Hulk began a rampage at Wayne Tech. Batman tossed a knock-out gas grenade at Hulk's head, and then kicked him in the solar plexus, forcing the Hulk to breath in the gas. Hulk went down quick.
Thor and Hulk have fought each other so many times, and almost never does one side truly beat the other — it usually ends up as a draw with both heroes standing (even if one is a little bit worse for wear than the other). But in 2001's Incredible Hulk Annual, Thor uses a sneak-attack lightning bolt to the back of Hulk's head, knocking the jade giant completely out, and Thor picks up his unconscious body like a giant green ragdoll. Of course, as Thor is flying along, Hulk wakes up and then beats Thor unconscious, but hey, a win's a win, even if it's short-lived.
Once in 1972, X-woman Polaris was driving around looking for fellow X-person Havok. The nearby Hulk spotted the green-haired Polaris, thought is was some girl he had a mack on named Janella, and grabs her Kong style. Havok shows up for a battle, and when the Hulk picks up an entire cliff face and perhaps to beat Havok to death with it, Havok uses his powers and just knock Hulk the hell out. Havok and Polaris leave the unconscious Hulk on the ground without a care in the world. Most embarrassingly for Hulk, this happened in his own comic.
7) The Thing
The Hulk and the Thing have probably fought each more than any two Marvel superheroes. While it's often a draw, usually Hulk wins out over his rival/friend/sparring partner. Except the one time in Fantastic Four #320, where Doctor Doom forced Hulk — in his less strong, more smart Mr. Fixit form — to battle the FF for kicks. The Thing was in a spiky, overpowered mode at the time, and the only thing that prevented the Thing from knocking out Hulk completely was Doom sending in a green Hulk robot for some reason. Even though he hadn't been KOed, the Hulk conceded he lost that one.
8) Doctor Doom
It's worth noting that Doom also more or less defeated Hulk at the beginning of FF #320 by scrambling the impulses in his brain until he agreed to help Doom retake Latveria (after annoying the FF, obviously).
9) The Fantastic Four
In Fantastic Four #166, the entire group got together to take out the rampaging Hulk: The Human Torch blinded him, Mr. Fantastic immobilized him, the Invisible Woman put a force-field around his head so he couldn't breathe, and the Thing delivered the final blow. In the Thing's defense, he felt like a total asshole about it.
The Man of Steel and the Incredible Hulk have tangled a few times in crossovers, none more infamously than in the 1996 Mavrel Vs. DC crossover series, where fans got to vote to see which company's character would win the fight. Pitting Hulk against Superman was a no-brainer, but it was also a no-brainer that Superman would win. Basically, the Hulk and Supes punched each other for a while. Superman threw the last punch. The end.
11) Namor the Sub-Mariner
Way back in Avengers #3, the Hulk makes the mistake of getting in the water to battle Namor, the prince of Atlantis. It's not even a contest, and Namor walks back on shore dragging the Hulk's unconscious body behind him. In Tales to Astonish #100, Namor hands the Hulk his green ass again (although he's under Pupper Master's control at the time). In the Incredible Hulk #118, Hulk and Namor played a game of underwater chicken and smashed into each other; Namor woke up, and Hulk reverted to a very unconscious Bruce Banner. As Hulk has gotten stronger over the years, Namor has had a significantly tougher time with him, and now most of their fights end up as a draw or a win for the Hulk.
The Hulk and Spider-Man have tangled many times, and it's usually all Spider-Man can do to not die during these battles (he usually concentrates, wisely, on luring Hulk to more sparsely populated areas). The Hulk has beaten Spidey a few times along the way. But Spidey can content himself with his memories of Amazing Spiderman #328, when he was imbued with the Power Cosmic. Sebastian Shaw had hired Mr. Fixit Hulk to kill Spider-Man, and Hulk challenges Spidey to a duel on Roosevelt Island. Spidey punches the Hulk into orbit. Orbit. Then, almost sheepishly, flies into space to retrieve him. Awesome.