Nikon SB-600 or
Sigma EF 500 DG STNikon SB-800 or
Sigma EF 500 DG superThe 4 flashes are very good quality. Nikon's are a little bit more expensive but they have a couple of more features. I think the SB-800 does have the strobe feeature and the DG super doesn't have it.
Read their respective technical features and if you have any questions, please ask them
Basically, the SB-800 can be installed, then measure lights and focus, then unplug and be put something else to flash wirelessly. All the flashed are same power if I can remember correctly but the more expensive ones just have more features..