Power Grid is one of those games that's so good, it makes you want to sell off half your collection, because any time you're playing another game you'll be thinking about how you could be playing Power Grid instead.
The mechanisms are so smooth, everything just works. The game has a natural catchup mechanism where if you've built the most buildings, you're buying supplies and building last, which makes everything more expensive, but if you have fewer buildings to power, you're making less money each round. It's really clever, and I'm always gobsmacked at how elegant this game plays out.