When solving go problems, there are some conventions that are commonly used. Some of them are not necessarily obvious, so it is probably a good idea to read this list at least once before hitting more serious tsumego-studies.
When asked for the best result in an isolated life-and-death situation, this order usually applies, assuming you are the defender. Reverse the lists, if you are the attacker.
This is the order of preference for different kinds of life:
Also note, that life by double ko counts as "unconditional life".
When the best ko is needed, this order should be easy to remember:
Also note, that the number of internal threats can be important, since it can in effect change the player who gets to take first.
Other kinds of ko (multi-stage ko, eternal life, etc.) do not fall in this list very nicely, so you have to judge their goodness in the context of the given problem. Generally however, a thousand year ko is better for the defender than a direct ko.
In an isolated problem it does not really matter what kind of ko threats are left behind, so all sequences that kill unconditionally are usually considered equal. The only notable exception is death by double ko, which gives an infinite number of ko threats, and thus has to be considered better than "just dead".
Discussion on order of priorities moved to /Discussion
See also: