As with all academic writing, it is important to select appropriate vocabulary. When writing a conclusion, there are a number of words or phrases you can use. The most common phrase is, of course, In conclusion. However, as it is the most common, it is also the most overused. If you are looking for a high result in your IELTS test, it is important to demonstrate that you have a wider range of vocabulary than other candidates, so it is best to avoid using the more common expressions. Better options are To conclude, in brief, to summarise, overall and therefore.