Which of the following thesis statements is strongest?
the water cycle is an important part of the weather system.
the water cycle begins with evaporation and ends with precipitation.
the water cycle begins when water is evaporated into the atmosphere; it condenses and freezes, then falls as precipitation.
the water cycle is a process that begins with evaporation, moves into condensation, and ends with precipitation.

I'd say that the last thesis statement is the strongest.
The water cycle is a process that begins with evaporation, moves into condensation, and ends with precipitation. 

The first thesis statement is the subject; it doesn't specify reasons why the water cycle is important.
The second thesis statement is weak; it doesn't state the purpose of your topic about the water cycle. 
The third thesis statement is too broad; it has too many details that a thesis statement shouldn't have.