Let's look at the Emil versus the Mk.I. Both had technical advantages and disadvantages over each other, the list is quite extensive, but context again. The Emil had, despite a high wing loading benign stall characteristics, but it was a beast on the ground; poor visibility, poor lateral control on the ground, a heavy tail, awkward undercarriage geometry which made ground loops a mandatory aspect of operations, particularly after landing on rollout. Taxiing with the canopy open was a no no because the vibration from operating from grass strips meant the hinges wore out. In its favour the Emil has cannon armament, high altitude, high cruise speed at altitude and operating as escorts from altitude was able to carry out vertical combat manoeuvres to nullify the advantages that the Spitfire and Hurricane had over it in being superior low speed dog fighters.
Emils also had variable pitch props, although the switch was initially located on the dashboard, which meant the pilot had to do a dance with his hands to activate it, a distraction in combat that led to the lever being mounted sensibly on the side console next to the power lever. The Spit I started the war with a giant block of wood on the nose, which to be fair was in the process of being replaced by a two-position variable pitch prop, which again was later updated with a fully CS prop. The German supercharger was a very efficient piece of technology that operated barometrically by varying its output based on its altitude, whereas the Merlin's supercharger was effective only at a prescribed altitude band.
In the BoB the Emil was formidable and widely recognised as having a better ceiling than the RAF fighters, but of course the way combat worked was that the German escorts would have to dive down on their adversaries. In commenting about the Bf 109's perceived superior altitude, one RAF pilot stated, "Well, they have to come down here and get us..." This meant the RAF fighters could take advantage of their virtues, better manoeuvrability, which as we know combat descended into twisting turning individual scraps where the superior dog fighters could get the better of the Bf 109s that couldn't pull as tight turns as its adversaries. At high angles of bank, those lift slats snapped open asymmetrically and spoiled the pilot's aim.
As for the claim the Emil was arguably the best fighter in the world in 1939, it could at that time out perform the Spitfire I in altitude and it was more modern - no wooden prop, cannon armament, plus the Luftwaffe tactics learned in the Spanish Civil War meant that the Jagdgeschwader were better prepared for modern war.