Mark's #1: The Settlers of Catan
Think about it - good game play requires more than simply a mastery of The Arcane Secrets Of Initial Placement; you also have to be able to trade (or not trade) wisely. Games are decided NOT by the dice (despite many people complaining otherwise)... they are decided by playing ALL aspects of the game well. Of course, there's the flexibility of the hex-based board system which keeps the game fresh as well.
Can you tell I like it... a little bit?!