Axolotl said:
Infinite Jest by David Foster Wallace.
A thousand times, yes. I don't think I've ever laughed as hard.
As for my personal recommendations, I've got a couple for you. These are actually my two favorite books, and while they're not necessarily super-long in terms of pages, if you invest the necessary time, they're plenty "insanely long."
The Divine Comedy by Dante Alighieri- It's commonly regarded as one of, if not the, greatest poem ever written, and that's not without good reason. By turns a theological and metaphysical discourse, traditional epic, political commentary, love story, and slasher film, it's nothing if not diverse, but never schizophrenic. If you can stomach an unrelentingly graphic torture scene (read: the entirety of Inferno), you'll be rewarded with the best and arguably most influential journey story ever written. Its beauty ranges from perverse to serene, but it never loses the central contrapasso elements that make the story a cohesive and arresting whole.
House of Leaves by Mark Danielewski- So first, a disclaimer: as a straight novel, House of Leaves is laughing in many ways. But what it succeeds at, that no other book besides Vladimir Nabokov's Pale Fire has ever come close to, is creating a book that is not a book. Imagine a Blair Witch Project-style horror story being told by five different people at once. Three of them don't exist, one of them doesn't speak English, two of them have never heard the story before, one of them is mixing up the story with the tellers, one of them is mixing the story up with another story entirely, and absolutely nobody can tell if it's a horror story or a romance. Now take these stories, cut them up, burn half of them, and rearrange them in random order, and have another person write an academic examination of the lot just for good measure. House of Leaves is undeniably confusing, and while it's occasionally overwhelmed by its own pretentiousness, it's an experience like no other that I'd recommend to any fans of horror, mystery, thriller, romance, puzzles, or even anyone who loves to read and is looking for a challenge.