The phrasing of your question offers a few problems.

I'll try to explain why. Mussolini was appointed Prime Minister in October 1922 and held the position continuously until July 1943. At the end of his tenure he left the Country occupied by a foreign invading army, pitched against widely superior forces and in no condition to continue the fight ; therefore soon forced to surrender unconditionally. The hopeless war had been started by him, essentially, and while others may have looked favorably to the war, no doubt exist that Mussolini had the last word on the issue. The war had also cost over 400,000 dead and depleted much of the nation's reserves. The former allies were already planning an invasion of Italy that would bring the Country into civil war for two more years.

Many of his initiatives had also disgraced the public image of the Kingdom throughout the world and would cast a lasting shadow on the entire nation in the future – not last the passing of racial legislation but, often somehow forgotten, Italy's supporting role in the despicable actions of the Germans in the occupied territories.

This would be enough to make Mussolini – by far – the worse Prime Minister Italy ever had. Nonetheless, let's look at the economics.

The overall conditions of the Italian people, while not abysmal like some accounts would suggest, did not improve during his tenure. Indicators show a moderate progress of the heavy industry and infrastructure but also display how this was essentially paid for by a reduced buying power of the working class – a policy which many deemed untenable in the long term. The most well known success of the Regime – i.e. the land clearings – were actually regarded as half a failure by their first originator Arrigo Serpieri (who unlike Mussolini had actually devoted half of his active life to progress of the agrarian masses) and failed to develop that synergy between private initiative and state guidance that the Regime had hoped for. While this was also an unfortunate consequence of the economical crisis hitting Italy at the time, the other economical/social reform – that of the corporative system – was resolved in a suppression of the workers' rights, that brought no significant, effective improvement over any liberal system at the time (as I mentioned, Italy did grow economically more than other nations, according to some indexes). A general analysis of the economical situation is beyond what I can put together here: in short Mussolini's economical policies were – to be kind – of average effectiveness.

But let's go back to the phrasing of your question. As Prime Minister, Mussolini damaged – irreparably – the institutions of the Kingdom of Italy, the entire parliamentary praxis within the Kingdom, that his predecessors had (more or less enthusiastically) fought for, starting with the Count of Cavour in the 1850s. Building a set of rules and customs to temperate the Statute and develop the Nation as a modern democratic system, that was destroyed by Mussolini, in a deliberate conscious manner.

If we were to judge Mussolini as a Prime Minister of a liberal state: he walked into the house, set it on fire and watched it burn to the ground!

Of course when I said that Mussolini was by far the worst Prime Minister in the history of Italy, I did not mean to suggest he was incompetent: Mussolini did not see himself as a Prime Minister in a liberal state. He wanted to make large renovations to the liberal state and one of those was to get rid, not only of the Parliament, but of the entire concept of parliamentary life, that is at the basis of any liberal representative system. His accomplishments went therefore against what you are likely looking for in a Prime Minister – which at least involves the preservation of the State and the institutions one is supposed to run.

But, let's ignore this for the sake of argument, and compare Mussolini's rule over Italy with the various liberal Government that preceded it. Let's pretend there was no war, no persecutions, confinements, murders, violence; and follow De Felice's summary in his discussion of Mussolini's effectiveness as a ruler.

It is a fact that Mussolini had the longest, largely unchallenged, tenure as a Government head. He also enjoyed a large, almost unprecedented, level of popular and political consensus at the beginning of his action – certainly larger than that granted to the various Giolitti, Crispi or Depretis. With all this, and taking into account the fact that his governing action took place over a longer time span than any other previous government, his Government would rank among the worst.

With all their flaws and limitations the various liberal governments were more effective as far as economic growth, social progress, extension of civil rights (of course), foreign policy, participation of the citizens to the life of the state. You can of course pinpoint short periods of crisis when the government's action was essentially stuck (Pelloux, Facta), but those governments lasted months, not 20 years; and even the most favorable look, reveals that Fascism was ultimately inadequate, falling short not only of our expectations for a government but of the fascists' expectations themselves. A fact betrayed by the growing discomfort of various prominent figures – Mussolini included – with the Italian situation in the late 1930s.