Which hotel across from Penn Station was not designed by Stanford White?

Hotel Pennsylvania is indeed the correct answer because it was not designed by Stanford White. Instead, it was designed by the architectural firm of Helme & Huberty and completed in 1919. It is notable for its location directly across from Penn Station and has undergone various renovations since its opening to maintain its relevance in the hospitality industry.

The other hotels mentioned have established connections with Stanford White. The Hotel New Yorker, for example, was constructed in the 1920s and is known for its Art Deco style, although it is not attributed to White. The Knickerbocker was designed by White and is famous for its historical significance and luxury accommodations in Times Square. The Roosevelt Hotel, also associated with Stanford White's legacy in architecture, was designed by the architectural firm Schultze & Weaver, with significant involvement from White's contemporaries.

Understanding the connections of these hotels to specific architects helps provide context about the architectural history and development of New York City, which is crucial for a sightseeing guide.