I have a client that is starting a loan website. They need to collect applicant's bank account data and social security. On the site there will be a form to submit this data which the client needs access to.
What is the most secure way of getting this data to the client?
Don't ask us. Ask whatever government agency is responsible for that in your country. What you are doing has in many countries heavily regulated minimum requirements regarding privacy, encryption and auditability and such. Or rather, have your client ask what requirements you need to implement.
Do realize that there usually heavy penalties (ranging from fines, loss of license to prison time) from not following the legal requirements.