Signature moves like Roar of Time (Excluding Blast Burn, Hydro Cannon and Frenzy Plant.) Metronome, Smeargle and Sketch don't count.
While Victini can learn the most signature moves, they are not signature to it. Zygarde is the current Pokemon with the most signature moves, being Land's Wrath, Thousand Arrows, Core Enforcer and Thousand Waves. Though Partner Eevee doesn't usually count for things like this, it has 9, being Veevee Volley, Bouncy Bubble, Buzzy Buzz, Sizzly Slide, Glitzy Glow, Baddy Bad, Sappy Seed, Freezy Frost and Sparkly Swirl.
Zygarde has the most signature moves: - Land's Wrath. - Core Enforcer. - Thousand Arrows. - Thousand Waves.
I'm sure this is right.
If you do not count Smeargle, then the Pokémon with the most signature moves is Vespiquen, with Attack Order, Heal Order and Defend Order. If you include special event moves, then the Pokémon with the most would be an Arceus with Roar of Time, Spacial Rend, Shadow Force and Judgment, and a Victini with Bolt Strike, Blue Flare, Glaciate and Searing Shot.
I'm pretty sure that is Victniy the most.
It have 7 signature moves of its own generation.
Followed by Kyurem - Fusion Flare / Fusion Bolt (Learns naturally) - Ice Burn / Freeze Shock (Learns naturally) - Glaciate (Learns naturaly)
Victiny is really lucky here, I never understood why he has that much options, but I wish celebi and manaphy had some signature moves too.