I may be able to help one this one for the highest:

My company has a Visa card with a $2,500,000 limit on it.

We're a health insurance company and have it in the event that we need to pay for services at non-contracted facilities in foreign countries that will let you basically die on a table unless they get up-front payment (i.e., many third world countries).

I only know about us having to use it one time when a member needed emergency surgery at some hospital in Central America and we paid the ~$220,000 bill by giving the hospital the credit card number over the phone. They weren't going to touch the person unless they had guaranteed payment.

I'll have to find out who the issuing bank is!