the roman republic was basically a corrupt oligarchy of aristocrats. not really a vary effective governing body.
Only towards the end of the res publica (yes, I am sticking with that), actually. About a century before that, things were actually running along pretty well. When it shifted from "I do this for Rome" to "I do this for me" is when it became real bad.
This was more to do with a shift in how the Senate was viewed and operated.
In the early-mid republic, magistrates and consuls answered to the Senate (which was effectively a very large directory), and legislation was handled by the plebian council. This worked pretty well, because the Senate was in fact not a very effective governing body, and while their authority could enforced militarily, most of the domestic matters were handled by the plebs.
In the late republic and empire, the plebian council was effectively neutered, the Senate took over legislation, and the Emperor inherited executive powers. This thrust the Senate into a position it was not designed for, and as it happens having a bunch of rich oligarchs and aristocrats deciding the laws is not particularly conducive to much other than feudalism. Successive emperors curbed the Senate's authority to varying degrees, but ultimately the accumulation of wealth at the top brought the whole thing tumbling down.
Overall, in terms of government, republic>empire. The real successes of the Empire were from capable leadership, and under that leadership empire>>>>>>republic.
E: typos