Strongest King? And by Niko you guys are talking about Crimson Rain? (Not good with names here
)
Well, my thoughts in the spoiler below:
Spoiler for Strongest King:
In Vol6 Rain said herself she's probably one of the weakest Kings, despite being Level 9 like the other Kings as well. Since the only way to level up at that point is to KO 5 other Lvl9s, their level is capped and at Lvl9 your level doesn't reflect your capability like other levels can (eg, weak = below lvl4, moderately powerful at Lvl6 etc). There's exceptions to every rule, and Accel World's "Same Level = Same capability" rule is no different to other rules. Other Level 9s are 1st Generation Kings, and Rain is 2nd Generation after Black Lotus killed off Red Rider. Her experience can't match the others, even she herself said the rest of them are Monsters compared to her.
I personally think Blue Knight is one of the most powerful Kings, holding one of the 7 treasures after all. Green Grande's the only King to have participated in all of the 4 battles against Chrome Disaster, barring that time with Cherry Rook, and in Vol9 tanked Sixth Chrome Disaster's strongest attack virtually unharmed (The Strife however did get scratched, 3mm large). From that, Purple Thorn probably also is up there with The Tempest in her possession, but due to her emotions getting the better of her (ie, Lotus killing Rider and holding a grudge against her) I'd say Knight is above her.
Lotus, I'd tentatively say 3rd strongest, below Grande and Knight.
Red Rider lucky or not... do remember what kind of attack he died to