Gunpla is the Greatest! is the fifth and final episode of Gundam Build Fighters: Battlogue.
One day, Tatsuya Yuuki visits Iori Models, where Sei Iori's family lives. Next week, Tatsuya will begin a tour of Europe as Meijin Kawaguchi. But he has heard from Mao Yasaka that Sei has modified his Build Strike Cosmos into the Build Strike Galaxy Cosmos, and before he leaves, Tatsuya wants to play Gunpla Battle against Sei's new machine. Sei eagerly accepts this proposal, and begins a one-on-one match against the new Gunpla that Tatsuya has built for his overseas tour.
At Iori Models, Rinko is busy dusting off the display cabinets as Sei puts on the finishing touches on a shield when Tatsuya enters the store in his casual wear. Tatsuya then explains to a surprised Sei that he's going on a tour around Europe as the Meijin, and though he doesn't say it outright, Sei knows he wants a match with him. With Rinko giving the two privacy in the Gunpla Battle room, Tatsuya reveals that Mao Yasaka told him Sei had modified the Build Strike Cosmos. Sei confirms and reveals the Build Strike Galaxy Cosmos. Tatsuya is impressed with the work, musing he must have also improved the RG System. Sei wonders if Tatsuya is going to use the Ballistic Zaku, and Tatsuya reveals the new Gunpla he made for the tour - A-Z Gundam. Sei is impressed, it's designed for maximum propulsion and firepower, and has Yajima Trading's new counter-particle paint. Seeing each other's new Gunpla caused both to be fired up for the battle, with Tatsuya wanting to fight not as the Meijin, but as himself. With everything set, the battle starts in space.
After connecting with the Build Strike Galaxy Cosmos' transformable backpack, Sei is taken off guard as Tatsuya races in in Waverider Mode, slips past him, then change into MS Mode and completely obliterate Luna II with a single beam shot. Sei realizes Tatsuya is giving him a chance to see his Gunpla's capabilities to level the playing field, and this is confirmed by Tatsuya himself. The real battle then begins, Tatsuya shoots at the Build Strike Galaxy Cosmos, but Sei blocks it using the shield's energy barrier. Sei counterattacks, but the A-Z Gundam is too fast, causing him to comment that it is six times faster, such is the skill of the Crimson Comet. As Tatsuya expected, Sei activates the RG and Star System simultaneously, meeting the A-Z Gundam in a clash of beam sabers. Breaking away, the two continue to clash, their vulcan cannons ineffective against each others' thick armor, leading to Tatsuya to use the A-Z Gundam's Prime Cannon Shoot attack. Sei blocks the attack, but the sheer power launches the Build Strike Galaxy Cosmos into the Earth's Gravity well.
However, Tatsuya rescues Sei, but only because he wants to obtain a complete victory. With the danger over, the pair resumed their fight in the Earth's sky. Sei pulls out his beam rifle, but Tatsuya shoots it out of his hand. Sei then kicks it towards Tatsuya, and when Tatsuya destroys it, he is blinded briefly. This allows Sei to race in and stab one of A-Z Gundam's beam cannon with a beam saber, destroying it. Tatsuya reacts by charging at Sei, and activating a beam blade from the nose of the A-Z Gundam's waverider mode. Sei counters with his beam saber, and the clash resulted in an explosion that took Build Strike Galaxy Cosmos' right arm. As the two Gunpla fell, Tatusya transforms A-Z Gundam into ms mode and grabs the Build Strike Galaxy Cosmos, preparing to piledriver it into the nearby island. At the last moment, Sei is able to boost the two of them away into the ocean.
The two Gunpla emerge from the water, and even though the Build Strike Galaxy Cosmos is in a terrible state, Sei decides to keep going, as that's what Reiji had taught him. The two go in for one last beam saber slash, but the Build Strike Galaxy Cosmos' right knee unexpectedly gives out, allowing Sei to dodge Tatsuya's strike and damage the A-Z Gundam in the waist. The two then cease operating and the battle ends in a tie. As the battle ends, Tatsuya is impressed, but Sei is displeased - to him, the draw is as bad as a loss and that the dodge was just luck. Tatsuya tells him that's how battles are and wants a rematch when he returns to Japan. Sei agrees and they shake hands. Sei then wonders if it was a good idea to wreck the A-Z Gundam so close to his overseas tour. Tatsuya admits it wasn't, and immediately got to fixing it at the shop. Sei realizes that he fought him to make the A-Z Gundam better, and described it as "rebirth from destruction", causing Tatsuya to wonder if he was trying to be witty.
- Tatsuya using the A-Z Gundam to rescue the Build Strike Galaxy Cosmos mimics other incidents in Gundam meta such as the MSZ-006 Zeta Gundam rescuing the AMX-004 Qubeley Mk-II, the Archangel rescuing the GAT-X105+AQM/E-X01 Aile Strike Gundam and others.
- The island where Sei and Tatsuya crash on is Cucuruz Doan's Island.
- The episode ends with a message revealing a new Build Fighters story and the date February 2, 2018.