Maybe most objective definition of 'overrated' is how often a given audience will misunderstand actual facts about a plane's operational history. Even if everybody has different opinions based on the same agreed facts, that's not as important as misunderstanding of fact. Which facts are misunderstood will depend on the audience, though.
I voted Spitfire with a reasonably well informed audience in mind, because there tends to be IMO the most stubborn refusal to admit to the often mediocre operational record of mid-mark Spitfires, and the relative lack of importance of the later mark Spits to the outcome of the war, even among such people. In first case there's still a strong tendency to quote Spit success in terms of Brit claims whereas in many cases in period ca. 1941-43 the results look quite different from the German side, or considering both sides' reported losses only. In the second case post 1943 the Spitfire was increasingly really successful in fighter-fighter exchange ratio but the real action v the German fighter force shifted to Germany and the Spit couldn't reach (until bases were established near the German border late in the war). The Spit's overall success in the BoB is a valid reason to consider early mark Spits successful and important fighters. Also, from a purely technical POV late mark Spits were excellent a/c compared to contemporaries. But again the rose colored (or often 'coloured'
) glasses on the plane-for-plane operational success 1941-43 and operational importance after 1943 is why it wins my overrated award.
On P-51, if someone's level of background and interest is limited to 'rah rah' individual a/c books that present *all* planes' operational records in the best possible light based on claims, and junky TV documentaries, then OK the P-51 is overrated, perhaps massively depending how junky the sources. But it's less overrated by refusal to accept its real record, among people who should know better, than the Spitfire is, IMO.
The Zero is claimed to be overrated compared to some monumental reputation...except very few people especially Americans or Europeans seem to credit it such a reputation. So I don't understand what it's supposed to be overrated compared to, in such an audience. Eventually a/c like the P-40 could meet it on equal terms in air combat...OK but the P-40 was basically a later airplane. And no other contemporary real fighter, certainly not the P-40 or F4F, could come anywhere near matching the radius of the Zero. That was a key factor in Japanese conquest (especially supported by *land based* Zeroes) of a pretty notable % of the earth's surface in just a few months; conquests it took the Allies, even with overwhelming industrial superiority, a few years to regain. Many discussions of the Zero fail to admit that at all or, 'OK long range, that's nice but...'. No, it wasn't just nice but a major influence on the early Pac War at a strategic level. The Zero is in the running for most *underrated* major fighter of WWII IMO.
I don't see the Bf109 as competing for either prize. There's seems a fairly widespread recognition that it had success in some periods/theaters of the war (or wars, WWII and Spanish Civil War), and didn't in others, for a variety of reasons. Like the Spitifre, pretty different a/c all called 'Bf109' were fielded over a long (in those days) period, but there seem to be fewer Bf109 fans who insist it was always a successful fighter across that whole period (which it wasn't) than Spit fans who claim the same (it also wasn't). Although there are people who do overrate the Bf109, as is true with *some* people for just about any fighter.
Joe