Best Free Company Action to spam?

The best action is dependant on the nature of your FC membership.

Levelling members alt class / new players:
Heat of Battle

End game raiders:
Meat and Mead and Proper Care (sometimes) repairs are cheap compared to Food / Potions cost.
Alternately, Reduced Rates
Back on your Feet is not worth it because usually when a person dies, the raid is wipe any ways which resets the condition.

End game gatherers:
Reduced Rates

Levelling crafters / gatherers:
Helping Hand / Earth and Water are very helpful but their value is rather limited as the best way to level those is use Leves.

The others:
A Man's Best Friend - almost useless if you have a house and stable, plus the weekly XP bonus for companions in the Challenge log.
Brave New World - completely useless, it takes like 40 minutes to go from level 1 to 10 on an alt class.
Live off the Land, What You See, In Control, Helping Hand - are situation-ally good if you have not finished your gear melding and can help you get over the hump.
That Which Binds Us - SB is so slow, that you hardly notice the difference.