This analysis looks at all generations (and National Dex) but puts a heavier emphasis on the current generation.
Depending on the format and the surrounding metagame, I would go with either Jirachi, Manaphy, or Victini.
JIRACHI:
Jirachi is famed for being an Serene Grace Iron Head flinch flinchy flinch machine, but it also has another set it can run: Z-Celebrate (ofc in formats where Z-Moves are legal, like Gen 7 or National Dex.) This set, along with using Stored Power and Aura Sphere for great coverage, also has Iron Head for the same nasty 60% flinch chance. Going back a few generations, we see Choice Scarf Jirachi popping up for, you guessed it, excessive flinching with Iron Head. All in all, Jiachi is a solid Pokemon, and even without Iron Head's nasty flinch, its great defensive typing allows it to shine in certain situations; for example, in National Dex Doubles OU, Jirachi is a great defensive Pokemon and potential Trick Room setter without abusing Iron Head.
MANAPHY:
Manaphy's power is derived from Tail Glow and the prominence of Rain teams. The moveset of Tail Glow, Surf, and Ice Beam can pretty much annihilate most everything, and then you can have the choice of Energy Ball for opposing Waters, Psychic to mutilate Toxapex, or Rest to couple with Hydration and heal back to full HP in one turn. Even without Rain, being able to get to +3 Special Attack and have a near perfect set of moves for the tier is insane, especially considering Manaphy's solid defenses at 100/100/100 allowing it to tank certain hits. Another setup option for it is Take Heart + Acid Armor, which has a more bulky setup approach and runs Scald for burn fishing along with Stored Power.
VICTINI:
Victini can run a few different movesets, all to great success. The first is leveraging its base 100 Speed stat and Choice Scarf to fire off fast V-Creates, acting as a great revenge killer. Victini also has access to Trick, allowing it to rid itself of the Choice Scarf against defensive Pokemon that really don't want to be Choice locked, or just opt for Zen Headbutt. Out of all the sets (and all the Pokemon listed, honestly) Scarf V-Create is the most immediately threatening. Victini can also run a decent Boots pivot set with U-Turn, and it has access to moves like Encore, Toxic, and Glaciate to disrupt the opponent. Finally, Z-Celebrate Stored Power is back again with Victini, and it functions similarly to Jirachi, minus the flinch chance and plus the possibility of running a move like Will-o-Wisp or Substitute to catch the opponent off guard.
All in all, my final decision is Victini, mostly because of the variety of movesets. Jirachi can be strong (and catch people off guard with Scarf Iron Head) but its reliance on flinching and lack of offensive prowess beyond flinch hacks makes it a bit luck reliant. Sure, it has U-Turn, but at that point Scarf Victini just exerts so much more pressure. Manaphy is a bit hit-or-miss depending on the metagame, as you pretty much require the +3 boost from Tail Glow, and even then there are hard counters. For example, Toxapex walls non-Psychic sets, Dondozo walls non-Energy Ball, and the Pink Blobs take all of its moves outside of Rain. For Take Heart sets, it can be easily pressured by strong super-effective moves before it starts setting up; plus, its lack of power before Stored Power boosts means it is easy exploitable with a Water resist if few boosts have gone up. Although Victini does have its own flaws, like the Choice lock from Scarf and weakness to Pursuit (in National Dex formats or in previous generations), Victini's explosive power makes it much more reliable on a game to game basis. Unlike Jirachi, who still requires some amount of luck, or Manaphy, who needs to set up safely, Victini's Scarf sets allow for immediate damage while outpacing many common Pokemon, such as Scarf Lele, Scarf Urshifu, Weavile, and more. Z-Victini is also a threat, and Fire and Psychic are deceptively good offensive typings for smacking everything for at least neutral damage (this isn't even considering a potential coverage move of choice, like Focus Blast for Heatran and Tyranitar). Finally, if the opponent was expecting one of the above offensive sets, you always have the option to use Boots and disruption moves to support your team. Victini's typing allows it to switch in on a plethora of Choice-locked Pokemon, such as anything from Lele, Close Combat from ScarfShifu, Leaf Blade from Scarf Kartana, and Triple Axel from Band Weavile. From there, Victini can do what it's supposed to do, maybe land a crucial Toxic on an incoming defensive Pokemon or U-Turn out to gain momentum. All in all, Victini's set versatility and reliability on a game-to-game basis makes it better than Jirachi and Manaphy, although there are definitely environments where the latter two are better. Manaphy in particular is downright monstrous if Rain is prevelant, and Scarf Jirachi is a good anti-offense Scarf pick if threats like Dragon Dance Dragonite need a soft check.
