What are some of the best on camera flashes for the x2d?
I have standardized on Profoto flashes. They have both on camera flashes and studio flashes. If you start with their on camera flash and decide to buy into their studio flashes, their remotes can control everything. The on camera flash can be great as a fill flash and can also be controlled remotely.
Agree with mikaelo67, Profoto works perfect with X2D, choose the Nikon version, my A2 + A10 works perfect on field. sometimes the c1+ can support the "macro" job as well.
I have been deep diving on A10 and A2 those are my next purchases.