Breda Ba.88 hands down if you want to consider it a fighter (was actually designed as a fighter bomber). From Wiki;
Two Gruppi (Groups) were equipped with the Breda Ba.88 on June 1940, operating initially from Sardinia against the main airfield of Corsica, with 12 aircraft on 16 June 1940 and three on 19 June 1940. The crews soon found that the Bredas were extremely underpowered and lacked agility, but the lack of fighter opposition resulted in them being able to perform their missions without losses.
Later, 64 aircraft became operational serving 7imo Gruppo in the North African Theatre with 19imo stationed in Sardinia, but their performance remained extremely poor resulting in the 7imo Gruppo being grounded from the end of June until September, when the Italian offensive against British forces started. Of three aircraft used, one was not even capable of taking off, and another could not turn and was forced to fly straight from their base at Castelvetrano to Sidi Rezegh.
With anti-sand filters fitted, a maximum horizontal speed of 250 km/h (155 mph) was reported in some cases and several units were even unable to take off at all. These machines were fitted with "Spezzoniera" Nardi dispensers (with 119 kg/262 lb bomblets), 1,000 rounds for the three 12.7 mm (0.5 in) machine guns and 500 rounds for the 7.7 mm (0.303 in) Bredas. Although the weapons were not loaded to full capacity and the aircraft was lightened by eliminating the rear machine gun, observer, bombs and some fuel, lessening the weight did not substantially affect the aircraft's performance. Such attempts to reduce weight failed to achieve positive results.
By mid-November, just five months after the start of the war on 10 June 1940, most surviving Ba.88s had been phased out as bombers and stripped of useful equipment, and were scattered around operational airfields as decoys for attacking aircraft. This was a degrading end for the new, (theoretically powerful) Breda Ba.88. This action forced the Regia Aeronautica to use totally outdated aircraft in North Africa, such as the Breda Ba.65 and Fiat C.R.32. As an additional problem, Regia Aereonautica remained without any suitable heavy fighter, a notable weakness for a major air arm.
Similar "heavy fighter-zerstorer" projects were developed in several countries. In France, the Breguet Br.690 even with only 1,044 kW (1,400 hp) was more capable than the Ba.88. Despite some problems of reliability, the Br.690 showed a practical usefulness that the Breda never achieved. It is notable that the Ba.88 was also a contemporary of the Messerschmitt Bf 110, with no great differences in hp, weight, P/W ratio or wingload. But the difference in success was immensely in the Bf 110's favour.
Niclot was the only pilot capable of flying this machine at its best (and only in the racer version which was much lighter), while the average pilot was not capable of using it effectively. Despite its impressive world records and aggressive, slim shape, the Ba.88 was a failure and totally unable to undertake combat missions. Its structure was too heavy, wing loading too high, and the engines were quite unreliable with insufficient power. The Piaggio P.XI was quite powerful, but never reliable.
At least the Buffalo, Defiant, I-16 and even the Me 163 drew some blood and was at least able to go into combat. It's pretty bad when you can't event take off to fight and in the end just pushed off to the side to be used as a "decoy"!!!!