Marius
-established reforms which lead to numerous civil wars that ultimately destroyed the Republic
-responsible for the breakdown in relations with Sulla which was causing Sulla's march on Rome
Sulla
-marched on, and then occupied, Rome twice
-revived the office of dictator, which hadn't been used since Second Punic War
Pompey
-often claimed victories of others as his owns, e.g. war against Spartacus, Sertorius, Mithridates, etc
-refused to renewing the alliance with Caesar after the death of Julia and Crassus, thus responsible for causing Caesar's march on Rome
Caesar
-killed one million, and enslaved an another million, of Gauls during his conquest
-pretty much destroyed the Republic, by appointed himself a dictator for life
Antony
-refused to recognize Octavian as the sole heir of Caesar, thus responsible for the fall of the Republic
-preferred Egyptian Queen, Cleopatra, than his Roman wife, Octavia, which then causing Octavian's invasion of Egypt
Octavian
-antagonized Antony to the point that the entire Roman Senate and populace turned against Antony himself
-abolished the Republic for good, thus setting the stage for all civil wars and dynastic disputes in the Empire
So, which one?
(I didn't include Crassus because I'm sure he will get the highest votes of all...)