Gomu Gomu no Jet Gatling on Lucci
Lucci, the first villain that Luffy fights who is physically as strong as him (yes because his previous opponents were all weaker physically and had the advantage on Luffy only because of weapons, poison, Logia abilities, etc.). Anyways, Luffy defeated him with pure strength like he should against a master of Rokushiki. Also, my personal analysis: Luffy and Lucci had opposite visions of justice (Lucci wanting to restrain Robin and Luffy wanting to free her from WG). They were battling in the tower of justice so when Luffy gets Lucci out of the tower, he wins the justice battle (my favorite fight in the entire series btw).
Gomu Gomu no Storm on Oz and Gigant Jet Shell on Moria
Luffy basically beats his own shadow, Oz, with Moria’s own powers.
Concerning Moria’s defeat I haven’t find anything interesting to mention.
Gomu Gomu no King Kong Gun on Doflamingo
This is a parallel with Crocodile’s defeat. While Crocodile worked underground and hidden, Doflamingo was seen by everyone and worked freely without any fear to get caught. So ironically, Luffy smashes this highly demon down to earth where nobody can see his face.
Gomu Gomu no Cannon Ball on Cracker
I see this fight as the same as the one against Blueno. Simply to show Luffy reached Emperor Commander’s level.
Gomu Gomu no King Cobra on Katakuri
I think it wasn’t the attack itself that defeated Katakuri. In my opinion, Luffy’s will to not give up got Katakuri down for the first time in his life.
Also that sign of respect from luffy at the end of the fight was awesome.