It's all about these two things: what kind of activities should that flash unit cover, and how much would you like to invest.
If you want to go cheap (but not to compromise the quality of photos) buy a "third party", with a guide number of at least 20 (20 meters at 100 ISO) with the "auto" modes (doesn't HAVE to be TTL capable), brands like Metz, Sunpak... This would be sufficient for "all around" activities. If you plan to shoot weddings and alike, you might gonna need somewhat more power, so choose the model with a guide number of 50 and upwards.
If the money is not an issue here, go for SB-800 to SB-910 - any of them will do.