SB-600 if you 'die die' want to enjoy Nikon CLS. Can be used on-camera, also can be used as CLS wireless slave and 'commanded' by the built-in flash on some Nikon DSLRs.
SB-700 if you want full CLS features and many more features, but cannot afford the SB-900.
SB-900 will be the work-horse for just about any shooting situation, versatility, power, CLS, etc.