Several Champions were given disgustingly high AP ratios on their ultimate since 1) they likely won't be building much AP (or any, or the ultimate was designed with old Trinity Force and old blue buff still around).
2) Even if the champion went full AP, they become nearly useless aside their ultimate scaling.
However with Sylas in the game, he can kind of get around both rules and abuse nutty ratios.
Here's a partial list:
Ashe: 600 magic damage +100% ap. / 300 + 50% secondary targets.
Hecarim: 350 magic damage +100% ap.
Zac: 350 magic damage +90% ap.
Malphite: 400 magic damage +100% ap.
Amumu: 350 magic damage +80% ap.
Rammus: 120 magic damage +20% ap for 8 seconds (doubled damage against structures).
Miss Fortune: 75% ad + 20% ap for 16 waves over 3 seconds (320% ap total).
These are absurd numbers for some abilities: to compare Lux Final Spark does 500+75% ap.
And those are just the ones I can think of, excluding stuff that's single target like Nautilus and Tristana.