Heatblast is really cool. He can generate and control intense heat, which makes him great for combat and for melting things. He has a flaming body which gives him a very intimidating look.
Fourarms is really cool. He's big and strong with four arms, which gives him great power in combat. Heatblast is also amazing as he can generate and control fire.
Gwen would be a great character to pair with Alien X. Her magical abilities could complement Alien X's powers in interesting ways. For instance, when facing a threat that requires both brute omnipotent force and the finesse of magic. Another good pairing could be Kevin. His ability to absorb and use different forms of energy could interact with Alien X's power source in unexpected ways, like when they need to combine their powers to break through a powerful energy barrier.
One of the most popular is likely 'Starship Troopers'. It has a great mix of military action and alien encounters. It shows a future where humans are at war with a race of bugs from another planet.
There are some really cool 'Ben 10 Alien Force Fanfiction' out there. 'The Mutant Invasion' is a story where mutants from another dimension invade Earth and Ben has to use his alien forms in unique combinations to fight them. 'Gwen's New Powers' is another interesting one. It shows Gwen developing some unexpected powers and how she and Ben deal with that. Also, 'Kevin's Redemption' is a story that focuses on Kevin trying to make up for his past mistakes.
I'm not entirely sure as it could be a very specific or fan - made story. But generally in the Ben 10 Ultimate Alien series, Ben Tennyson has a new Omnitrix which gives him access to ultimate forms of his aliens. The big story might involve him facing new villains, protecting the Earth and the galaxy, and dealing with the consequences of using his powers.
Some of the alien characters themselves, like Heatblast or Four Arms, are also popular. Their unique physical characteristics and powers make them interesting to write about. For instance, Heatblast's ability to control fire can lead to all kinds of scenarios, from battling fire - based villains to using his heat to solve environmental problems in a story.
One common theme is friendship. The female alien often forms a strong bond with Ben, and they support each other through different situations. Another theme is power. The female alien usually has some unique power that plays an important role in the story.
I'm not entirely sure about the exact plot details of 'ben 10 ultimate alien the big story'. It might involve Ben Tennyson using his various alien forms to face some big threats and challenges, but that's just a general guess.