Excel. Before it got subsumed and was infected with the ribbon. The Excel team were truly talented developers.

I think much of that was part and parcel out Microsoft acquiring them when they bought Excel, and it's obviously degraded in the interim, but for a while they were all island of programming excellence in the Sargasso Sea of usual Microsoft incompetence. The only thing Microsoft is really good at has ever been marketing and sales, and they weren't even good at staying on the ethical, legal side of business operations.