Sukhumvit is, in many ways, the center of modern Bangkok and not only one of the longest roads in Thailand, but one of the longest roads in the world. Towards the west end you have Ploenchit Road and Rama I Road, but to the east it runs most of the way to the Cambodian border!
The lowered-numbered sois are densely packed with shopping centers and hotels, including the gigantic Siam Paragon and the dense warren of Siam Square opposite it, while the upper sois are a popular residential area for resident expats.