Jolly. Unless you are gonna do some sort of mixed set on it, jolly is your best bet since you don't want to reduce your attack because earthquake is a physical move and you have a strong grass stab in wood hammer. Torterra's spa is also significantly weaker compared to its atk stat so jolly is the second best option. If you do decide to use a mixe set, go with naive so that your Torterra can take more physical hits and you don't really want to eat special hits anyway
Having said that, you might want to consider a +atk nature since Torterra is slow anyway so the extra power could come in handy. If you do decided this, use adamant nature
