If you are asking about which pre-MBA qualifications is ideal for a post-MBA investment banking career:
Well obviously investment banking (IB) analyst or private equity (PE) experience is ideal, but I know many people who have switched from strategy consulting, corporate finance, and other finance fields to IB or PE associate after their MBA. If you don't have pre-MBA IB or PE experience, then you should keep up with global business, economic, and capital market landscapes as best you can on your own. Keeping up to date and having adequate knowledge of those three things is essential to performing well in your interviews.
Some great resources that you should often read and utilize are the Financial Times, Wall Street Journal, Bloomberg, and CNBC for daily and real time news, and The Economist and Bloomberg Businessweek magazines for weekly editorial analysis.
Taking and passing the CFA exams, or at least CFA Level I, also usually helps greatly for your industry switch.