When writing a conclusion for task 2, I always start with the words "In conclusion". There's no reason why you should learn any alternatives.
Here are some phrases that I would not use:
- All things considered
- To sum up
- In summary
- To summarize
- In short
- In a nutshell
- To put it in a nutshell
Phrases 1 to 5 are acceptable, but I still wouldn't use them myself.
Don't use any phrase containing the word "nutshell". 6 and 7 are not appropriate for an academic essay.