Who won the Quidditch Cup in Harry's first year?

Question

Who won the Quidditch Cup in Harry Potter's first year at Hogwarts?

We know that Gryffindor didn't win it, as Oliver Wood says in Harry's third year:

“This is our last chance — my last chance — to win the Quidditch Cup,” he told them, striding up and down in front of them. “I’ll be leaving at the end of this year. I’ll never get another shot at it.

“Gryffindor hasn’t won for seven years now.”

— Prisoner of Azkaban, chapter 8 (Flight of the Fat Lady)

The popular assumption seems to be that Slytherin won the Quidditch Cup, as they were also set to win the House Cup that year until the last minute.

However, we know that even in years other than Harry's first, such as the 1985-6 school year, the Quidditch Cup winner (Gryffindor) and the House Cup winner (Slytherin) were not the same house.

The only match results that we know about in Harry's first year were the ones in which Gryffindor was involved:

• Gryffindor defeated Slytherin 170-60 (Philosopher's Stone, chapter 11);
• Gryffindor defeated Hufflepuff by about 150 points (based on the extremely short match duration; Philosopher's Stone, chapter 13);
• Ravenclaw defeated Gryffindor by the most points any team had defeated Gryffindor by in 300 years:

Wood wasn’t pleased.

‘Now, listen here, you lot,’ he said, glowering at them all, ‘we should have won the Quidditch Cup last year. We’re easily the best team. But unfortunately, owing to circumstances beyond our control …’

Harry shifted guiltily in his seat. He had been unconscious in the hospital wing for the final match of the previous year, meaning that Gryffindor had been a player short and had suffered their worst defeat in three hundred years.

— Chamber of Secrets, chapter 7 (Mudbloods and Murmurs)

From these results, we know that Ravenclaw may have had as many as 3 wins -- including a win over Gryffindor which featured a margin of victory which was certainly many hundreds of points -- whereas Slytherin and Hufflepuff can only have had as many as 2 wins each.

Finally, in the event that both Ravenclaw and Slytherin (as well as Gryffindor) finished with two wins, we know that the Quidditch Cup is awarded based upon the teams' margins of victory in their matches. Ravenclaw's 300-years-historic margin of victory over Gryffindor is likely to have given them a decisive edge in this tie-breaker.

Isn't the most likely outcome that Ravenclaw, not Slytherin, won the Quidditch Cup in Harry's first year? Is there any other evidence that points to a particular house having won the Quidditch Cup in that year?

Answer 1

The Information We Have Is Insufficient And Contradictory

There is no definitive answer, mostly because the issues of how the Quidditch Cup is awarded and how Quidditch wins contribute to the House Cup are controversial.

1. There simply is not enough information, most games and match scores are not reported.
2. To make matters worse, JKR is not very good at math - as she admitted herself - and there appear to be some inconsistencies.

Depending on the ranking system (aggregate points; matches won + point difference; some hybrid?), sometimes it is more likely that Slytherin won, sometimes it is more likely that Ravenclaw won. This "Almanac of Quidditch at Hogwarts" gives a great overview.

Answer 2

There's no definitive answer that I can find in the books. However the Harry Potter Wiki states quite clearly that the winner of the Quidditch cup for Harry's First Year is in fact Ravenclaw, what they base that on I'm not sure at all.

Answer 3

We can guess Slytherin because they almost won the House Cup, and winning Quidditch adds a large amount of points to your house.