What mothership sends down a orbital strike the fastest?

All motherships charge at the same rate (provided you have their charge stat equally leveled), so how fast they charge depends on how active you are in the battle. A lot of the activities that charge your titan bar and earn honor also charge motherships, but not all of them. Unlike the titan bar, motherships do not slowly self-charge so long as you have bots on the field.

The following actions charge your mothership:
- dealing damage with your robot
- getting kills or assists
- capturing beacons (you still get credit if you enter a beacon that is already being captured by someone else)

The following actions do not charge your mothership:
- healing (yourself or teammates)
- sitting idle
- dealing damage via means other than your robot (drone-mounted weapon, motherships strike, etc)
- for corrosion weapons, only direct damage charges motherships, meaning you do not get bonus charges for the corrosion
- low-end one-shot weapons like Trebuchet or Nashorn struggle to charge motherships (weapons like Gauss and Reaper do much better for some reason)

As you can see, motherships reward more aggressive, damage-focused playstyles over beacon-focused or conservative playstyles (like using sniper weapons). I am not a huge fan of this approach since such a playstyle just makes you fodder for spenders and tankers if you commit too much to it. Anyway though, I hope this helps, as motherships are kinda weird.