When justifying yourself as the "best fit" for any role, you should take two things into account:
1. How your skills and experience suit the position
2. How your personal qualities and ethos fit in with the organisation
The first part is relatively straightforward, you just need to describe the skills you have acquired in previous employment and say how they will help you to succeed in the job you are applying for.
If your previous position was the same or very similar - no problem.
If they are a little different, you might need to pick a small aspect that was similar, and exaggerate it a little.
For the second part, how you will be a good fit for the organisation, you will need to do some research to find out what qualities they value as a business and try to relate that to what they value in their staff. In banking this might be, for example, reliability, integrity etc.