I think this question is a bit difficult to answer because it depends on what you consider important. A word that I consider very important and useful for language is the verb to be. Without it, you could not have written your question (you used both is and been) and my answer would have to be formulated very differently (if it were at all possible). Without it, we could not describe things using adjectives without giving more information. For example:
- The ball is round. (This is a complete sentence)
- The round ball …. (part of the sentence is missing)
There are also a lot ‘be expressions’ (phrases that contain the verb to be, such as to be able) that we would not be able to use.
Just in this short answer alone, I use some form of it 13 times.
You can learn more about ‘to be’ here: The Most Important Word in the English Language and Its History
Edit: For this answer, I assumed we were focusing on the English language as the question is written in English and posted in English-speaking Quora.