The game that introduced euro games to me, and resulted in a closet full of games. This is still probably the most commonly played game in our collection and with good reason. You don't need the expansions to keep it interesting. It's perfect even by itself.
The gateway drug to european games. Whenever I am trying to introduce people to euro games, I always start with Ticket to Ride. Easy to grasp and addictive, it becomes a great entry point to introduce games like Catan.