The one that uses the fewest resources. The easiest to write. The most general. That is, we consider how long the algorithm takes to run. It is therefore important for us to be able to analyze the running time of our algorithms.
Different inputs lead to different running times.
Not all operations take the same time. For example, addition is typically quicker than multiplication, and integer addition is typically quicker than floating point addition. The same operation make take different times on different machines. The same operation make appear to take different times on the same machine, particularly if other things are happening on the same machine. Many things that affect running time happen behind the scenes and cannot be easily predicted.