Brick for the win
In the original game of Settlers of Catan (not counting any of the major expansions - Seafarers or Cities & Knights), brick is the most valuable resource. There are only 3 brick and 3 ore tiles while there are 4 of every other type. While ore is necessary for cities (and you need at least one city to win), brick is necessary for roads and settlements, both of which are the cornerstones of early expansion, and ultimately winning.
Seeing as how there are less brick tiles, and it has a larger demand than ore ,it is the most valuable resource on average and you should remember this while trading. Toward the end of the game, however, it's value tapers off. You should keep this in mind when trading early on in the game. Finally, whie this strategy is generally applicable for most games, you should ignore it when there is a huge surplus of brick. Just remember that overall brick is the most useful resource out of the give.