Ben Tennyson (Classic)/Ultimate Alien

In Ben 10: Ultimate Alien, 16-year-old Ben's secret identity was revealed to the world by a fan of his named Jimmy Jones, and he was now a megastar superhero adored by the public. This series featured Ben's new car, the Mark 10, gifted reluctantly by Kevin in Ben 10: Alien Swarm. Ben was now armed with the Ultimatrix, which evolved his aliens into their Ultimate Forms.
The series introduced two new enemies for Ben's rogue gallery: nasty news reporter Will Harangue, who saw Ben as a public threat, and a villainous Osmosian named Aggregor, who drained the powers of five powerful aliens from the Andromeda Galaxy; Bivalvan, Galapagus, P'andor, Andreas, and Ra'ad, as part of his plan to obtain the Map of Infinity and travel to the Forge of Creation to become the most powerful being in the universe. The five aliens also provided Ben with five new transformations: Water Hazard, Terraspin, NRG, Armodrillo, and Ampfibian.
thumb|Ben meeting his 10-year-old self
In The Forge of Creation, after Kevin was mutated by absorbing the Ultimatrix and lost his sanity again, he successfully defeated Aggregor with help from Young Ben. 
Kevin was cured in Absolute Power: Part 2 with help from Cooper, Darkstar, and what was left of the Dominus Librium. 
Sometime after Absolute Power: Part 2, Ben and Julie broke up, or in Julie's words, "might as well have been broken up". However, after rescuing Baz-l in Eye of the Beholder, Ben and Julie became a couple again as he promised to be a better boyfriend.
In The Creature From Beyond, Ben's team faced the Lucubra, a creature from an alternate dimension.
In Solitary Alignment, Ben learned about Azmuth's creation, the Ascalon, the sword that was wielded by Sir George. He, along with Gwen and Kevin, was told about how Sir George obtained the sword. Ben struck up a deal with Sir George, that if he was unable to slay Dagon, then he would take the sword and attempt to defeat Dagon himself.
Sometime after Ben 10,000 Returns, Ben appeared in Ben 10/Generator Rex: Heroes United in order to help Rex deal with the sentient Alpha nanite. Ben was at a TV interview when a strange storm picked him up and dumped him into Rex's universe, where he mistook Rex for an alien attempting to destroy Bellwood. Rex mistook him for an EVO and tried to cure him. Both found that they were not what the other thought they were. After Agent Six was injured in an explosion after destroying Alpha's mechanical host, Ben was taken back to the Providence defect group's headquarters. While in the base, Ben learned of what was going on in that world, becoming confused by some facts, but generally grasping that things were not going good. Deciding to team up with Rex, Ben met Caesar, Rex' brother and learned that he had banished the Alpha nanite to the Null Void, thinking it was an empty dimension when in reality, it is a prison for alien criminals. However, Alpha returned and succeeded in absorbing Rex's Omega-1 Nanite to finally create a body that could sustain itself. In addition, it created and briefly used a replica of the Ultimatrix to battle Ben and Rex. Eventually, Ben transformed into Upgrade and fused with Rex, allowing them to use more powerful versions of Rex's builds to fight Alpha. With Caesar's help, Ben entered the Null Void as a stop on the way back home, using Murk Upchuck to dump Alpha into the Null Void.
In The Ultimate Enemy: Part 1 and Part 2, Ben and his team participated in the final battle between Dagon and Vilgax. Dagon turned every human on Earth into an Esoterica and fought Ben and George. Dagon's seal was broken, and he appeared in the human world. Dagon was too strong for even Ultimate Way Big to stand against, and Sir George was turned to dust when trying to fight him. Vilgax used the Vox Peniculus to drain Dagon's very essence to transfer his power to him. The team took the sword Ascalon and fled to the Plumber base in Mt. Rushmore. Vilgax forced his way inside and knocked Gwen and Kevin unconscious. Vilgax boasted about how he planned to fight another of Ben's Ultimatrix transformations. Ben decided to use a different means to fight him, Ascalon. He uses the sword to subdue Vilgax and absorb Dagon's power from him.
 
Although Ben won the battle, he was tempted by Vilgax to fulfill his desire for peace and justice with a simple thought to destroy all evil. Ben considered this at first, but Gwen, Kevin, and even Julie tried to talk him out of it. He then used the power to turn every Esoterica on Earth back to normal, and all the free will that went with it. He relinquished the sword, saying that Azmuth was right, it was too much power for anyone to have. Azmuth appeared and Ben gave the sword back to him. Azmuth asked for the Ultimatrix too and gave Ben a new Omnitrix, one that he had been working on for six years since he found it. Ben asked for its Master Control, to which Azmuth replied perhaps for his 18th birthday.
16-year-old Ben also appeared in a flashback in Rules of Engagement where he was in his room playing Sumo Slammers. He started talking to Julie, but he also started yelling at the game at the same time. Julie, believing Ben had broken up with her, turned off her phone and the flashback ended.