Just like the question on what aircraft was the best of WWII, telling which machine was the worst of the war, is an extremely difficult, if not impossible, question to answer with accuracy.
What of the parameters applied to answer the question? There can be as many as there are individuals thinking about it.
I would narrow my search to those machines who saw action in important numbers, leaving aside those which saw service in such small numbers they can even be considered as prototypes. (Still, I would never ever consider the Komet Me 163 as the worst, not at all; it was so revolutionary it still had to solve its problems, however, that would never place the Komet nowhere near amongst the "worst".)
Let the show begin:
For instance, the PZL polish fighters which faced the Luftwaffe during the invasion of Poland in September, 1939, strictly as machines were not that bad.
The problem was they were TOO OLD to still remain in service. Even worst, to see service against an extremely modern and tactically advanced foe such as the Luftwaffe equipped with cannon armed, radio equipped single-engined Bf109´s.
Still, a few Polish pilots, very few, made it airborne and shot down a small number of German planes, bombers mainly.
The same can be told about the soviet Polikarpovs, the I-15 biplane, the I-153 and the I-16 (Ishak). I repeat: strictly the machine, they made excellent planes when they were conceived. Likewise, they suffered the same fate of the Polish fighters when they met the very modern Bf109´s.
Those old soviet types had excellent maneuvering capabilities, but were very miserable at both climbing and diving, their speed was low.
Those 3 tiny soviet fighters were TOO OLD to be in service. Obsolete. Outclassed. Outpowered by the Luftwaffe´s single engine fighters.
Still a few Soviet pilots managed to score victories against the Luftwaffe flying those 3 planes. However, the bulk of the Polikarpovs got effectively blown out of the skies or destroyed at their bases by the superior planes and tactics of the Germans, still a few of those remained in service until 1943.
The soviet LaGG´s, which were far more modern than the Polikarpovs, were extremely mediocre and unreliable, and the Germans as well sent countless of those plummeting down to the earth.
PARTIAL CONCLUSION: (STRICTLY THE MACHINES) the Polish PZL´s the Soviet Polikarpovs while being very good planes, should have had an earlier retirement. WWII as it started, was not the business of such aging and obsolete types and their losses were frightful.
The LaGG´s, while including very interesting features made very bad planes. Totally unreliable. As a whole it can be said the LaGG-3 was a failure.
The RAF with the turret equipped Defiant made a contribution for nurturing the "worst plane" thread. Yes, it scored a few victories during the initial stages of the Battle of Britain, but it got immediatly outclassed in combat by the Bf109´s and losses were very high.
The USAAF with the Brewster Bufalo also is awarded a medal. It had nothing to do against the Japanese Zeros.
I do not think any of the German planes deployed in signifcant numbers to front units qualifies at all in the "worst aircraft" category.
Some obsolete types like the Henschel He123 biplane, were deployed in very small numbers for night harrasment missions, but in very small numbers, and such attackes were frequently succesful.
If you think of the Me210, which failed, it was a prototype being tested, which eventually lead to the excellent Me410 which brought the Luftwaffe back over England in 1944.