One of the upmyielts IELTS examiners answered it.
Without doubt, Task Achievement. Let’s look at why.
Your English level is basically marked by your Coherence, Lexical and Grammar score but Task Achievement is all about answering the question.
Task 1
So many candidates miss a key feature in Task 1 and then they automatically get awarded Band 4.0 for Task Achievement. This goes for both the Academic and General exam. This doesn’t happen for the other parts of the marking criteria.
Now, if you get awarded Band 4 for Grammar or Lexical Resource, then your English is very low-level. It means you are making mistakes that make it difficult for the examiner to read and follow.  However, it is very rare to see Band 4 for Lexical Resource or Grammar in the exam.
Task 2
If you don’t answer the question fully then your Task Achievement score is capped at Band 5.0; even if your English is perfect. So, if you are asked to discuss both sides and you only discuss one side of the argument then Band 5.0 is as high as you can go for Task Achievement. If you have misunderstood the question and what you have written is not answering the question then you are awarded Band 4.0.
The only other thing that compares to this is if you don’t write in paragraphs and then your Coherence score is capped at Band 5.0. This rarely happens these days though.
If you have questions about the exam then send them to us here, of via our Facebook page and IELTS examiners in the UK will answer them.
If you want an IELTS examiner to assess your writing and show you what you need to do to get higher marks then you can order that here.