Well, that depends on the rest of your team, really. Lucario also learns Dragon Pulse, which makes a great dragon counter with Lucario's dragon resistance (due to steel type), but Psychic is a good move overall.
STAB or not, Flash Cannon (Power 80, accuracy 100) is kinda redundant on Lucario. Steel-type moves are only super-effective against Ice and Rock types - these types are already nicely covered by the more powerful Aura Sphere (90 base power, NEVER misses) - the only Pokémon that might give Lucario issues in that category are ghosts and psychics (which you're more likely to use Dark Pulse against anyway). Flying types and bug types can be taken out using another move that Lucario has. That thing is a major special powerhouse - pretty much every special attack you throw on it will be over nine thousaaaaaaaaand.
My suggestion is ditch Flash Cannon for Dragon Pulse, keep Psychic and Dark Pulse. You might need them.
If you already have a dragon counter in your team, though, consider Water Pulse in case they pull a random fire-type on your tail before you can react. Not that that's very likely considering the lack of fire types nowadays, but you get the deal.