Which of the GoF patterns could you not live without?

Question

If, in future, you were only able to use a single one of the canonical patterns defined in the Gang of Four book, which one would you choose and for what reason?

Answer 1

Not a design pattern, but to me this principle was definitely the most influential on my programming style of the whole book:

Favor object composition over class inheritance.

Combined with the other principle "Program to an interface, not an implementation", most design patterns follow naturally.

Choosing one pattern as the most important is in my opinion impossible. They all have their purpose and complement each other. Its like asking if a hammer or a screwdriver is more useful. It depends on the problem, sometimes you have a nail and sometimes a screw.

Answer 2

Abstract Factory Pattern or Template Method Pattern... but mostly Abstract Factory Pattern. They have just been extremely useful on a lot of situations.

I specially liked the way Abstract Factory allows you to prepare for situations where you know your program may change in the future by following the same rules in a different way.

Is mostly like the Abstract Class allows you to define specs "inside" the code, restrain the code to follow that specs while the factory allows you to "choose" among those specs. Simply beautiful.