Victini is a perfectly solid choice. It has a massive movepool, is fully capable of using and abusing low-accuracy moves thanks to its ability, amazing stats when you get it that hold up throughout, and can go mixed - which is useful ingame. I never found its Psychic type to be a hindrance, and if nothing else I really was thankful for its fighting resistance. Plus the biggest reason Psychic types suck competitively is Pursuit, which isn't an issue because a.) its rare, and b.) who switches ingame? Fire Blast, Thunder, Focus Blast, and a psychic attack are all easily available and all you need.
Which isn't to say Darmanitan is a bad choice - because he's not. That attack is wonderful and he's fast enough to use it. But ingame I prefer duotype Pokemon for one thing, and Darmanitan is pretty fragile for another.
