Ticket to Ride

Alan R. Moon's train route-building game won the 2004 Spiel des Jahres and has sold over 10 million copies. You collect colored cards and claim railway routes across a map — simple enough for a 10-year-old, strategic enough that adults play it competitively. The genius is in the blocking: do you build your route efficiently, or do you spend turns cutting off your opponent's path from Montreal to Miami? The digital version on Steam has another 5 million+ players. Europe, Nordic Countries, and Japan expansions are all excellent.