It really depends on what features you want the flash to have, and what your budget is.
the sb910 is a great flash, at about $500.
If you want TTL and HSS, your options will be more limited than if you want a flash to use on manual.
for TTL and HSS you can get a sb600, sb800, sb700, or sb900/910
you can also get a yongnuo YN568EX for $180 new that does TTL and HSS
there are also a few cheaper yongnuo flashes that are TTL, but do not do HSS.
for manual flashes....the skys the limit really. too many to list.
Right now, I have SB24, SB28, and a few YN560's for manual flashes, (great for OCF)
and SB700, and half a dozen YN568EX's for TTL/HSS flashes.
Budget will most likely determine your best choices.