Hmm, if we aren't talking about severity of the 'crime' my initial reaction was to go with Marius. His several consecutive consulships along with running in abstentia certainly set a precedent. However, that's a rather simplistic view that doesn't take into account the reasoning, or the necessity of his actions (at least in part).
I do agree with Clodius that the Senate holds a great deal of guilt, but for me, that body must be broken down into its various factions and time periods in order to do this on a comparable basis. With that in mind, I have to choose Sulla, not only for marching on Rome but for assuming a lengthened stay as dictator and completely tearing down 500 years of Republican constitutional evolution. As much as the later figures did to circumvent the 'constitution' it was a terribly worn down instrument by that point, and Sulla's behaviour stands out to me as the most non constitutional.
Still, this is actually a very interesting question that requires further thought. Keep in mind the actual question about the constitution, not about right, wrong, or what political affiliation you may or may not agree with.