Question: What are the best rules variants for Monopoly?
Answer: The first thing you should do is to make sure you're playing by the official rules -- many people don't, and I think these rules make the game much better.
If you're not satisfied, check out our complete collection of Monopoly variants.
Mark Jackson's Game Central Station contains my favorite single list of good Monopoly variants. Game Central Station describes numerous ways to "Germanize" Monopoly -- that is, make the game shorter and more strategic.