Sure Ahri may be safe, but her catching tool is only decent. Any champ can take and use glacial to make their catching better.
Safe, and high impact, then I'd say supports are way better than Ahri in midlane. Anivia, Xerath, Leblanc, Lux, Zilean, are all safer than Ahri and each one has a pretty powerful cc tool. So, by your definition of "best" then you need to look at deathrate of every champ and the champ with least death is the most poweful. Although, the mistake here is, the amount of damage output a champ can deal.
So def, damage + safety + utility is the best combination for the best soloq champ. At least that is my def. Why? With damage you can kill key targets, and can carry further. You can kill the enemy fed carry, and if u ate safe and have utility, then you are def the best.
Like I said, for me, what best is, has the highest potential of carrying games and can output the most damage quite easily. Champs like Syndra, Ziggs, Lux, and many "other mages" provides this much more than Ahri. But you are looking for safety and high impact, then supports are the best champs in game. They dont provide damage, but very safe, and have one of most impacting abilities in game.
If supports were the best champs, then they will be played everywhere. I mean, it might actually work, but you will lack damage. Though, if you want to get carried through elo, sup is the easiest role to do it from, so I guess according to you, sup is the most powerful role in the entire game.