The most overrated military of all time is — without doubt — that of Sparta. It took them over twenty years to defeat Athens in the Peloponnesian War, and only managed that due to (a) Athens arrogantly turning former allies into client states, none of whom were exactly happy over this turn of events; (b) lots of naval help from Persia; and (c) [arguably] a traitorous betrayal from the aristocratic bloc within Athens itself.
The “War Nerd” doubtless sums it all up better than I ever could, in his review of the execrable “300”.
The second most overrated military of all time? I’d have to say Nazi Germany. They seemingly ran out of useful ideas sometime in early 1942, wasted resources on crackpot nonsense like the V-1 and V-2 rockets, neither of which could carry more than a bucketful of actual explosives, and don’t even get me started on garbage like the “King Tiger” tanks, especially since they actually had a useful tank with the Panther.
Of course, the fact that Hitler basically checked out mentally at some point, that he let his subordinates run amuck building little empires of their own and so on didn’t help much. Instructive to note that the one place Stalin didn’t interfere with his generals with on the tactical front, while that was the one place Hitler actually did continue to keep sticking his beak in. And Stalin would have tolerated the chicanery of a Goering or Himmler for about twelve seconds, before they’d have found themselves with their hands tied behind their backs and bullets in their temples.
Sidenote: Clearly the most successful fascist regime of all time was the USSR under Stalin. The moment the magic wand of “socialism in one country” got waved the USSR certainly ceased being anything Marx would have recognized. (Though IIRC Engels started making some very odd pronouncements toward the end of his life.)