I've had this internet handle since the last century. Most people in here are talking about Hyperion but Simmons was a fantastic cross-genre author. My favorites were his historical fiction that contained a fantastical bent:

Drood: Has Wilkie Collins as an unreliable narrator, depicting the last five or so years of Charles Dickens' life.

Crook Factory: An FBI agent is sent to Cuba to keep an eye on Ernest Hemingway, hijinks ensue.

The Fifth Heart: Henry James and Sherlock Holmes team up to solve a mystery.

The Terror: Tells the story of what happened to the HMS Terror that attempted to make the northwest passage. The Arctic is a character in itself in this amazing story. I thought the TV mini-series was fine.

Abominable and Black Hills: I haven't read these yet but look forward to doing so.

Honestly, I think Dan Simmons is my favorite author. I know his politics became unpalatable but I could never find it in myself to care. My heart sank when I saw this post.