What is the building at the beginning of each show?

I don't know where the Veridian building as seen from the outside is, but the building as seen on the inside of Veridian's office suites is a high floor in a real 12-story skyscraper on the Los Angeles Center Studios lot in downtown Los Angeles. Whenever you see the skyline outside Portia de Rossi's office window (facing east), that's the real deal! The most noticeable landmark one can see is probably the Westin Bonaventure Hotel, just across Interstate 110 from the building. Presumably, other sets used for series interiors (Phil and Lem's lab, for example) were likely created on soundstages at ground level.

The outside is the Americas Tower at 1177 Avenue of the Americas (at the corner of 45th Street and 6th Ave) in New York City.