GOAT? Assessing that is probably too complex, a combination of historical context, capability, and skills/tactics of the pilots who flew the planes.
The Sopwith Camel and S.E.5a were very capable. The Fokker DVII and DVIII were so good that, IIRC, Germany was specifically required by the Treaty of Versailles to surrender every one of them.
The F6F reigned supreme in the Pacific Theater of WW2, but many of Japans best pilots had been killed by the latter half of 1943, when the F6F went into action. And while Japan had some better fighters than the Zeke and Oscar, factories were too busy building replacements for losses to fully switch over to producing the newer models and their newer engines in significant quantities. And Japan's pilot training programs were incapable of fully training sufficient replacement pilots. So how much of the F6F's record was due to its capabilities and its pilots' skills, and how much was due to Japanese deficiencies?
While the F-15's performance is very good, what may set it apart is its role flexibility and upgrade capacity, which have led to its ~45-years-and-counting longevity. OTOH, while its raw performance is lesser, the F-18 could, arguably, be assessed as equal or better, since dual roles - fighter and attack - were
by design, and there is the additional role of the EA-18 Growler (has the USAF replaced the F-4G and EF-111? or have ECM been partially grafted onto F-16s and F-15s?).
