I'm not going to order them, because I don't really feel like it right now but champions I think are on the stronger side are:
Ziliean - Sure, in game he's not super powerful, but mastery over time isn't something that should be treated lightly, according to his lore an entire army was scared to bring him into battle so they just waited till he was too deep in meditation to fight back. That being said, Zil's powers are tempered by the fact of his Chrono Displasia (Think that's what it was called) and his peaceful nature, so in terms of raw power, he's probably high, but if we're ranking on an overall willingness to use that power, he'd be on the lower end of the 10.
Maokai got a hexakill when he first came to life in the TT (Or at least he did, idk how his ret-con worked) so he's pretty powerful.
Swain is clearly has an immense amount of power, not so much for his abilities (Even though he does have powerful magic) but because his supposed tactical brilliance and the fact that he's currently leading all of Nox. which gives him military might as well. He'd definitely be up in the top 5 if not top 3.
Xerath is strong, but I don't think we can say he'd be without his Sarcophigus, because he currently has that on him and the only people who could remove it, most likely never will.
Kassadin clearly is powerful, any mortal that can embrace the void without losing his sanity has strength of will, but I don't know about top 10.
Kayle probably would be up there, she's an immortal warrior from a land that's been at war for ( I think) Eons, which means she's probably a master swordswoman on top of her divine abilities.
Jax is so OP he had to be nerfed in the cannon too, so he's pretty beastly. That being said, idk if his abilities could take the others in a non-league match. It's fairly certain that the League of Legends is fought differently than outside, specifically that all the champions are brought to an even playing field. So when the odds are even, Jax can come out on top, however when dealing with the full might of some Champions, I don't think Jax would be as successful.