One Piece has so many iconic fights that at this point in the series, there are at least ten or twenty that are arguably the best. Luffy is the star in most of these clashes, but there are plenty of others, both within and outside of the Straw Hat crew, who have had their moment to shine.
However, there are also a lot of battles that were on the brink of happening, yet never actually came about. Many of these fights would've been inconsequential to the plot, but others had the potential to alter the course of one of the main characters, or possibly even the world of One Piece itself.
10 Smoker vs. Dragon
Following Luffy's miraculous escape from the execution stand in Loguetown, Smoker comes face to face with Dragon as he races to catch the Straw Hat captain. Upon learning that it was Dragon who was causing the weather to tip in the Straw Hats' favor, Smoker is in disbelief why the leader of the revolutionaries would help such a lowly pirate.
Later, the audience would learn that Dragon is Luffy's father, bringing the whole mystery to light. Knowing he's unable to take on such a foe by himself, Smoker chooses to chase Luffy further instead of fighting. It probably would've been very one sided, but it would've been interesting to see Dragon fight seriously.
9 Luffy vs. Garp
Luffy and his grandfather, Monkey D. Garp, meet while the crew is at Water 7. At first Garp is lenient, letting his grandson go for the express reason that they're related, but he's quickly reversed by the government. Now ordered to capture his kin, the Straw Hats are yet again forced to flee from their foes. Much later, Garp smacks Luffy down at Marineford, but no brawl takes place. Luffy and Garp have come close to brawling a couple times but have never quite got there. At this point in the story, it would be one of the coolest duels of the series, an unparalleled clash of armament Haki.
8 Eneru vs. The Navy
Had Eneru escaped and not been defeated by Luffy in Skypiea, his plan to conquer the land below would've proceeded as planned. This would've eventually lead to a clash with the Navy, and the amount of firepower needed to take Eneru down would've been extensive.
Luffy was only able to beat the Kami usurper due to his Devil Fruit completely nullifying Eneru's. In truth, Eneru was frighteningly strong, his Devil Fruit powers on par with a Navy Admiral's or Yonko's. A duel between him and Kizaru or Aokiji would've been flashy and intense.
7 Arlong vs. Jimbei
Arlong and Jimbei actually did get into a fight after Fisher Tiger's death, a fight which Jimbei won handily. However, a second brawl almost occurred between the two after the Warlord heard of Arlong's rampage in the East Blue. He ignored his former crewmate due to his newfound position, which resulted in the devastation of Nami's childhood village. If Jimbei had reined in the sawshark fish-man, Nami's backstory could've been entirely different.
6 Ace and Luffy vs. Akainu
Had Luffy not been caught off guard by Ace's vivre card being dropped, Ace's death might've occurred at a different point in the story. As a result, both Luffy and Ace would've been forced to fend off Admiral Akainu at the same time. Having never had the chance to legitimately fight together, the resulting battle could've been both flashy and meaningful. What audiences got instead was a moment of self-sacrifice, an instant that changed Luffy's course and showed him just how weak he was compared to the top brass.
5 Luffy vs. Burgess
After Luffy fought his way through the preliminaries of Dressrosa's Corrida Colosseum, he's reunited with his other brother thought dead, Sabo. Sabo promises to get Ace's Flame-Flame fruit, the prize for winning, and takes on the other combatants.
Eventually he wins, defeating Burgess in the process. Had Luffy and Sabo not swapped at the last moment, Luffy would've taken on Burgess instead, his first fight with a Blackbeard crew member.
4 Zoro vs. Mihawk Pt. 2
Less of a fight that didn't happen and more of a fight audiences are unsure happened, Zoro's training with Mihawk is a bit of a mystery. Zoro had improved leaps and bounds from their fight at the Baratie, and after defeating the sword-wielding apes he's taken under the Warlord's wing. It's assumed that the two have sparred and fought seriously multiple times because of this tutelage, but it's doubtful audiences will actually get to see the fights in question. That will likely be saved for a later arc.
3 Caesar vs. Smoker
During the Punk Hazard arc audiences got to see just how much stronger Luffy had gotten against Logia Devil Fruit users. Caesar's body, made entirely of gas, was no match for Luffy's Haki in a one-on-one duel. But it would have been interesting if Smoker, who was also on the island, had been the one to fight the mad scientist instead. The power of the Smoke-Smoke Fruit versus the power of the Gas-Gas Fruit would've made for a crazy fight, a duel to determine the stronger gaseous Logia fruit.
2 Fujitora vs. Doflamingo
While there was a three-way standoff between Fujitora, Doflamingo, and Law, there were a few moments where the Navy Admiral nearly took on the ruler of Dressrosa. His gravity-altering abilities seem more than up to the task of defeating Doffy in direct combat, as he's beaten pirates like Jack, who's worth 1 billion berries, without breaking too much of a sweat.
Fujitora's decision not to simply take Doflamingo out after he attempted to destroy the entire country is probably due to Oda wanting Luffy to be the one to beat him, and while that's understandable, audiences lost an opportunity to see a Navy Admiral actually go all-out.
1 Luffy vs. Blackbeard Full Fight
There have been multiple instances where Luffy and Blackbeard have been on the verge of clashing, from just before Skypiea to more recently in the New World. Fans have speculated that Blackbeard is going to be the final antagonist of the series, which would explain why the their fight has been teased for so long. It would also fit with the fact that Blackbeard is seemingly accumulating Devil Fruit powers for himself, making him an ability-filled juggernaut of an opponent.
The small brawl they took part in at Impel Down only served to heighten the anticipation, since at that point, Blackbeard only had his own Devil Fruit, and the battle was stopped by Jimbei before any real combat took place. While it may not happen for a while, the final clash between Marshall D. Teech and Monkey D. Luffy is certainly in the works, and will be a battle fans will remember for a long time.