The answer to your question would be that the most important sentence in a paragraph is usually the first one.
The first sentence in a paragraph is generally the topic sentence. The topic sentence summarizes the main idea. What is more, it is usually known as focus sentence as it organizes an entire paragraph. The fact is that the topic sentence, as I've said, acts as a kind of summary which offers the reader an insightful view of the writer's main ideas for the following paragraph.