For me Kingdom Builder was love at first sight. What really makes it tick for me is that it's a game of spatial intelligence, like Ricochet Robots for example. You look at space and try to analyse it then move ahead to find the best possible action within restrictions.
There are very few games that test your spatial intelligence and logical analysis at this level with simple rules and great variety to boot.
That's what I like about Kingdom Builder too, do you have any favourites that work the same way?