The Guggenheim Museum is Frank Lloyd Wright's only major New York building, and it remains genuinely radical 65 years after it opened. The exterior — a continuous spiral of white concrete expanding as it rises — is visually distinctive to the point of controversy. The interior, a single continuous ramp circling a central atrium, is either ideal for viewing art (you flow continuously through the space) or completely wrong for it (the slanted floor makes viewing difficult). Both positions are defensible.
The permanent collection covers Impressionism through mid-century abstraction — Kandinsky, Klee, Chagall, Picasso — and the special exhibitions are consistently major. The building tends to overwhelm the art it contains, which may be the point. On Fifth Avenue's Museum Mile, a short walk from the Metropolitan Museum.