Two good choices would be Wuthering Heights by Emily Bronte and Jane Eyre by Charlotte Bronte. While The Castle of Otranto was the novel that kicked off the English Gothic genre, you might find it difficult and rather boring. It was written in 1765, which makes it quite old. The Brontes wrote in the 1800s.
Wuthering Heights is probably the more truly Gothic of the two, but both contain strong Gothic elements. Wuthering Heights offers the classic Byronic hero in Heathcliff, who is one of literature's more notable leading men. The haunting setting of the English moors is prevalent in Emily's work, and the supernatural element is definitely stronger in Emily's work than in Charlotte's.
Jane Eyre has a lot to recommend it. The heroine is more likable than Catherine from Wuthering Heights. While Mr. Rochester is not as tormented as Heathcliff, he definitely qualifies as a Byronic hero. While the moors are not as important in Jane Eyre as they are in Wuthering Heights, the large country house with the forbidden wing presents the classic Gothic setting perfectly. Although the supernatural element isn't as strong, dark secrets, mental illness, and tragedy make up for that.
Either Wuthering Heights or Jane Eyre would be an excellent choice. Both are classics that have been written about by scholars and critics for decades. Either one would be interesting and provide many angles from which to write your research paper.