Curse is the best choice, as Torterra won't be outspeeding much even without the Speed drops. The extra Defense can help it set up more Curses, giving it more Attack, which can help it sweep opponents more easily.
Swords Dance is a good choice if you think Torterra is sufficiently bulky and needs Attack more than it needs Attack + Defense. It can be more effective, but Curse is typically better due to the bulk offered.
Growth/Work Up are bad because you don't have any special moves.