<p>Depends upon the field.</p>
<p>I’m in structural engineering, so some of the top design places are Skidmore Owings Merrill (tall buildings, particularly), Arup (known for working with “starchitects”), Thornton-Tomasetti (kind of a domestic version of Arup), Walter P Moore (sports stadia, particularly), and LERA (tall buildings, particularly).</p>
<p>Two of the top forensic firms are Exponent (general failure analysis) and Wiss Janney Elstner (building systems failure analysis).</p>
<p>There are a lot of good firms, but there are some who are arguably the “best” at something.</p>
<p>This is by no means an exhaustive list of the prestigious firms in my field, though.</p>