If loyalty counts, Nadella is a front-runner. He joined Microsoft more than 20 years ago and currently is head of the company's cloud computing, data center, and enterprise division. It's a highly profitable but largely invisible area that Microsoft is counting on for future growth. Combine that with the fact that he's well-known inside the company, and he may be a serious contender.
Investors cheered news that Microsoft CEO Steve Ballmer will retire, but there is no obvious successor in line.