It depends on what set you’re running. Generally I’d say to go with Timid to outspeed more, (I always prioritize speed over damage) but for a defensive set you’d probably want to go with Modest. I’ve even seen some Rash mixed attacker Volcanions that carry EQ.
I think one the most important things in Pokémon is experimenting. Switch between Timid and Modest, and see which one you’re inclined to.