Hopkins vs Trinidad. Hopkins was a 3-1 underdog and everybody expected him to try and rough him up on the inside like he often did prior to fighting Tito. He executed a clinic and showed all the world what it took to beat Tito.
Leonard Vs. Hagler. Sugar Ray fought hard the last 30 seconds of damn near every round and pulled off the upset.
Hagler vs Hearns . Hagler knew Hearns was the better boxer , hit harder and could go 12 rounds. Hagler turned the bout into an all out street fight , and we know the result. The only way he could beat Hearns.
Ali's strategy on Foreman vs Ali was brilliant in all it's layers. Not just how he fought the bout itself, but everything surrounding it. Ali mobilised the whole local population for his strategy, unheard of then and almost impossible nowadays. Such a shame there never was a rematch, but it's by far the "Greatest gameplan" ever.