It depends on what you want to achieve.
Vi and it’s descendants are brilliant editors for a programmer but not for writing prose. So stay away from them. ;-)
Do you want just to write text without being distracted by an overwhelming gui or are you fine with the hint at options?
Do you want to write in a terminal?
How much do you want to format while typing? By typing the format commands into the text or by clicking on buttons or ctrl-key magic?
Do you need version control?
For each of your combination of answers there are different solutions.
If you used pandoc hen you were programming your thesis. ;-)