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Arcanine is a superior bulky physical fire attacker, because of intimidate, morning sun, and extremespeed. Darmanitan is a better purely offensive physical fire attacker, because of sheer force and 140 attack.
Victini's niche is when you want a good physical fire attacker and your team also needs certain specific things that Victini can bring, such as a fighting resist or a specific set of attacks (Victini's movepool is stellar).
Honestly, the rock weakness upsets me more than the ghost/dark weaknesses, since Victini is best used with a choice item and thus has his bulk squandered by sneaky pebbles.
I still really like Victini, though - mostly its design. It's my favorite of the 100-in-all-stats pixie-looking legendaries.