the neatest place would be under the headlight unit as per factory.
advantage is that all the wiring and ballast are hidden and will be really neat. downside is that if any plugs are loose (not unheard of) you have to remove the bumper to fix. Also if you are in doubt over fire safety of ballasts etc, might be better to keep them in view.
I mounted mine on the front panel on my old one. knocked out the clip holding the wiring to the front panel on the drivers side and bolted the ballast into the hole (bend the ballast bracket 90 degrees, ballast will be standing up)
On passenger side, unbolt the bonnet strut bolts and bolt the ballast there.