It's hard to say. Ainz Ooal Gown has a vast array of powerful magic spells at his disposal. However, Ben when he transforms into Alien X could be extremely powerful as well. Alien X has reality - warping abilities. But Ainz has his intelligence and strategic thinking. So it could really go either way depending on the situation.
Well, it's a really tough call. But some might say Goku from Dragon Ball is the strongest. He has the ability to reach extremely high power levels through intense training and various transformations. His Saiyan blood gives him an edge, and he has fought against some of the most powerful foes in his universe and come out on top. Plus, his Kamehameha wave is a very iconic and powerful attack.
Ainz Ooal Gown is of course a very important character. He is the main force driving the story forward with his plans for the New World. Albedo, his loyal subordinate, also plays a significant role, always being by his side and helping him in various ways.
I'm sorry, I don't know who the author of 'Sliver Overlord Novel' is. There are so many novels out there and without more information, it's difficult to determine the author.
In Overlord 17 novel, some new characters from the New World might also become important. They could be leaders of kingdoms or powerful adventurers. Their interactions with Ainz and his forces add new layers to the story. For instance, a new character could be a wise king who tries to negotiate with Ainz, or a powerful mage who challenges Nazarick's power.