Although they collaborated for only a few years, the careers of Louis Sullivan and Frank Lloyd Wright were forever entwined. Clearly, I greatly favor Sullivan whom I consider the far more talented and romantic and who more successfully merged nature and architecture.
There are not that many clear collaborations between the two, but perhaps the most terrific is the Charnley Persky House at 1365 Astor Street in the Gold Coast, essentially Chicago's version of the Upper East Side. Today it houses the Society of Architectural Historians. Unfortunately I was not free on the days they did tours during my visit but got to see it from outside which is glorious in its own way.
Many debate how much is Wright and how much is Sullivan. I think the lines of the overall structure are clearly Wright, and the ornamentation is obviously Sullivan. One of the key things I got from my Sullivan tour given by George Paldo of Chicago Savvy Architectural Tours is that Sullivan is unique in designing soffits, and there is no example quite like Charnley.