Olaf, easily. Undertow is ~90 damage ability with a slow on a 2.5 second cooldown if used properly. Combine that with Berserker Rage and you potentially have one of the most powerful steroids at level 1.
Honorable mentions would be Darius or Trundle, they're not Olaf levels of strong at level 1 but they're up there.
I would definitely agree with Olaf. I currently play him in the jungle, and invading with him with a Approach Velocity gives you so much kill pressure level 1. Combo it with Ignite (in toplane) and 1 overstep by the enemy means first blood.
Honorable mentions would be Darius or Trundle, they're not Olaf levels of strong at level 1 but they're up there.
Darius, Sion, and Aatrox are pretty powerful as well, but not as much as Olaf.