Hulk #255 some time back--and here's another: The Mighty Thor #489, "Hel and High Water" Written by Roy Thomas, pencils by M.C. Wyman, inks by Mike DeCarlo. (We actually saw a panel from this issue years ago, checking out Sif's not-great costume!)
In exchange for Hela not invading Midgard (earth) with her legions of dead, Thor has agreed to join her in Hel as her slave. His Asgardian friends Balder and Sif were forced to swear not to come get him, as were the High Evolutionary's Godpack; so they enlist the help of someone who recently got out of Hel, and swore no oaths not to get Thor: the Hulk! Of course, this was the intelligent, "Professor" Hulk, from Peter David's run on the book. The Hulk also gets an answer as to why Red Norvell was Thor the last time he was in Asgard; since the real Thor had been exiled.
After Balder holds off Garm the Helhound, the Hulk makes his way in, and tells Thor he can either come willingly, or be carried out. Thor declines, claiming to do this of his own free will; so it's fight time. In fairly short order, the fight demolishes Hela's castle (she's annoyed, but has eight more...) and the Hulk realizes he's at a disadvantage this time: although intelligent now, if he gets too angry, he'll go all "Hulk Smash!" but turn into Banner! (This was a fail-safe to keep the Hulk from rampaging, and pretty effective, really.)
Meanwhile, on earth, the Godpack and Kurse catch up with dark elf Malekith the Accursed. Although Kurse really, really wants to kill Malekith again; he takes the high road and returns him to Hel. Hela isn't impressed, since she has Thor now; but the Hulk/Thor brawl may demolish even more of her realm. Kurse volunteers to stay in Hel (since with Malekith dead, he didn't feel he had a reason to live) and Hela accepts.
Thor and Hulk return to Midgard (after a brief stop to punch out Garm) and Thor resolves to stay on earth, saying farewell to Sif and the Godpack. This was just a couple issues before Warren Ellis's brief, shot-in-the-arm run; so a lot of plotlines were either wrapped up or dropped here.