A weird one, but Provoke. Green has been pretty bad in terms of creature combat lately, and fight spells are basically useless in constructed. I think Provoke is pretty balanced nowadays, given that it's dependent on attacking and there are already so many value engine permanents that killing 1 creature a turn (excluding the turn it comes down) is strong but not unbalanced. Tacking on a condition or mana cost could also tone it down for lower rarities/mana values).
My more conventional pick would be Persist, lots of design space and it's fun to get extra use out of your creatures.
