Gideon S said:
somewhere I have an old "Elseworlds" (I think, it may have been "What If") comic where the Hulk was intelligent, strong, and completely evil. In fact, he was so powerful that he had killed all of the heroes of the Marvel Universe, and wore Wolverine's adamantium skeleton as an arm-band, Doctor Strange's cloak as a sash, and Captain America's shield across his forearm...
I'm pretty sure the story you are thinking of is
Future Imperfect, a 2-issue prestige format story by Peter David and George Perez -- excellent story, well worth reading (and recently reprinted by Marvel as a single-volume trade paperback, if I recall correctly).
While the Batman / Hulk encounter was indeed an Elseworlds/What If story,
Future Imperfect is not. It is part of the Hulk's actual continuity. Historians place its events between issues somewhere in the early 400s of the
Hulk's comics series (something like between 410 and 420) if memory serves.
In the story, the Hulk winds up in the future (or
a future) and encounters "the Maestro", who is as you have described. (As we know, there is more than one possible future -- see
X-Men: Days of Future Past, etc., for more of the published scientific research on this.)
Events from
Future Imperfect later had effects on the Hulk after he had returned to the present (as detailed in his monthly comic) (but I have no recollection of which issues).
What is open to dispute is whether the events in the Hulk novel
What Savage Beast are real or fictitious. The Maestro plays a major role in that book. It's written by Peter David, so whether or not it's historically accurate it's still a very enjoyable novel, well worth searching for.
(As I recall, Peter David intended for the novel to be part of official continuity. It features a major event in the Hulk's life, which probably would have had major repercussions on the Hulk in his comic book if Marvel had not replaced actual accounts of the Hulk with fictional ones during the Heroes Reborn year.)
To sum up: Batman knocking out Hulk,
not real. Hulk becoming evil, wiping out all the heroes and taking over the world,
real (at least in one possible future). See? Truth is as strange as fiction!