Not Sulla since, though a faithless and slaughterer politicien and dictator, he was the one who reestablished the oligarchic republic for one more generation. Sulla was an optimate : if he triggered 2 civil wars, it is because he did not want to be ousted from the first circle of power and prestige he had fought so hard to reac again (after 2 centuries of disgrace for his family).
To have the republic become empire, you need a very popular politician who is also a very good general or who has the absolute loyalty of very good generals of modest origin. And you also need this guy to have the alliance of a majority of the italia-non roman cities which are the main reserve of manpower for the armies.
Marius was too old for this.
Livius Drusus was an optimate and not enough popular outside Rome : see Badian for this who demonstrated that Drusus failed because a majority of italians opposed his plan to ... take their lands for the sole benefit of the old romans, rather counting on Marius to give them the roman franchise for free.
Scipio Aemilianus was an optimate.
But a good choice would be Gaius Gracchus. Have him better manoeuver in 122/121 so that he makes some kind of truce with his optimate enemies.
He remains the darling of the equites and of the italian ruling classes, so there is no room left for Marius to take this position (set free by Gracchus's death).
Have then Gracchus become consul and then assuming the same role of the popular and victorious military leader against Jugurtha and the Cimbri and Teutones that Marius was in real history.
And this will be a very strong starting base.