Which of the 5 resources in Settlers of Catan (base version) is most important? Why?
This will depend tremendously on the situation, but it's worth learning some generalities:

If you are playing with just the base set:


brick and wood are completely identical as everything that needs one wood also needs one brick (roads and settlements). 
Wheat is completely superior to wool/sheep as it is also needed for cities in addition to everything wool is needed for.
Wheat and Ore are about tied as you need 1 wheat for a settlement and no ore and 3 ore for a city and 2 wheat. My experience is that wheat is slightly better as the ability to continue building settlements is usually worthy a slightly diminished city building capacity caused by an increase in wheat and decrease in your ore.


Anyone who has played 4+ games of Settlers knows that you need a variety of resources to win, although there are widely differing ways to get them, so there's no such thing as 'the best' resource as the distribution is so situational to the island layout, what other players have, what development cards and resources you have, and what everybody needs. However, wheat is essential to virtually any strategy, ore is needed for a city/development card strategy, wood and brick are necessary in at least small amounts for any expansion via new settlements and roads, and wool is overall probably least useful. 

That is, until nobody goes for  wool but you, making you the one who can build settlements faster and extort ridiculous prices from your adversaries for it until you take an early 3 point lead for a decisive win. As I said, it's all relative. :D