Did John Belushi really drink a whole bottle of Jack Daniels in Animal House?

The directors & producers have said consistently it was just iced tea.

That said, there have been rumors floating around since the '70s that it was real JD, and that they either deliberately filmed it at the end of the day or that they had to send him home after filming that scene.

There's no way to know with certainty, and that's part of the magic of the movies. I think it's a lot more plausible that some people just want to believe one more story of unbelievable excess from a life of unbelievable excess. There's really no compelling reason why the movie's creators would deny it if it were true: everybody knows the he consumed lots of drugs very recklessly and died young as a result.