I think that each part of the speech is important because it has functions specific.
Maybe, the most important part is verb because it describe an action. And this shows to occur. For example, they walk together every day. If I don´t use the verb, people don´t understand the sentence.
Other part interesting is the conjunction. The conjunctions are used to connect. I don´t remember that there were many conjunctions.
For example, although, unless, as long as, even if, etc.