The Kernel Team or Core UI teams would be fun. But based on my qualifications more likely Exchange, Biztalk, or even Office.
But frankly I am being disingenuous if I said I wanted to work for Microsoft. If the pay was right and the job was good, I absolutely would, but it would just be another job. While Microsoft does some very good stuff, I'm not sure how much I would enjoy the middle management culture within the company and the politics of it all.
Google would be higher on my list. Apple would be much lower on my list.