According to reliable sources, Duquesne Incline on Mount Washington has the best vantage point of Pittsburgh you can get. There's a 180 degree view of Three Rivers intersection, where the Ohio, Monongahela, and Allegheny Rivers meet. You can see Heinz Field, where the Steelers play, and PNC Park, where the Pirates play, nestled between the skyscrapers of downtown and the bridges that span the rivers.
It's actually a tiny museum with historical memorabilia and pictures of early 1900's Pittsburgh when the smoke from the steel mills was so heavy that it looked like night 24 hours a day. There's also a cable car that takes you down the side of the mountain. Duquesne Incline is a free and awesome way to get introduced to the city of Pittsburgh.