Difficult choice...
My personal choice (using my heart, not my mind) would be Mithridates Eupator (Greek : ). This half Greek - half Persian King of Pontus (very greek at ideology and culture ) was Rome's terror (something like the Soviet so called super-power for Americans) for 2 generations of Romans. They constantly defeated him and he tenaciously regrouped and stroke back. When he died, Rome was celebrating for months. Mithridates of Pontus was a thorn in Rome's side for forty years before Pompey earned his title "the Great" by defeating him. He was the last of the ancient "Greeks" (ideologically).(Cleopatra excluded). (see also : http://www.pontos.dk/round_table_mithridates/TBNmithridates_ personality.pdf)
From the given answers :Hannibal was the greatest general and (using my mind now) the greatest danger for Rome. But he was very early and Rome at that time wasn't the super-power empire that we now refer to. (see map :red=roman territory)
Spartacus was the greatest threat because slave revolt was the economic - social - military -political nightmare for the Empire. He emerged from nothing, he had not a state to support him, and he (also) nearly made it.
The others mentioned were insignificant by comparison.
I think (given all that...) I will vote for Spartacus.
Edited by Noir