Depends what you're looking at. ATT is known for its cell phones and such - where Microsoft is known for its computers.
How is powerful judged? Microsoft could turn millions of computers worldwide into a huge bot-net, seems kinda powerful. ATT could cripple a lot of the telecommunications in the US (I guess).
Yall must be kidding. The ones of you who said "cellphones" evidently doesn't know ATT at all! ATT is GLOBAL. They own several other communication companies in other countries under other names. They are one of the major internet backbones (as in carries most of the internet traffic via their lines). If ATT goes down, most of the internet would go down, also. If Microsoft went down as far as operating systems goes, you could use a Mac or even Linux.