Rylai used to be a niche item
For bruisers like rumble/Vladimir/Elise
For champions that didn't have cc like mordekaiser
For Mages that needed to survive against
and other bruisers
For supports/mages that wanted to bring more utility/kiting power if their spells 
it was not built on
Azir never built it the champion spotlight, watch every lcs games of 2015 he never built it, he also needed cooldown reduction but that's an other story.
Karthus needed tons of AP and mana to convert it into AP with arch. staff, rylai passive also sucked as hell with his kit until riot changed it.
syndra/xerath/annie/viktor, all of those champs already had utility but have now even more utility while now they don't even need to build damage anymore since they can one-shoting easy targets thanks to the free 100 AP (
)
Riot olafed every other mage items, luden echo has a shit cost efficiency, no way to not build rylai now
V4.13:
Ability power increased to 100 from 80. Health reduced to 400 from 500.
revert this shit immediatly, return rylai to what it was: a bruiser item.
: 100 ap
: 100 ap
: 120 ap
: eventually 100 ap
: 80 + minimum gained from passive = like a bit over 110 ap
Spammers only.
treatment, less dmg, more health. Champions like 

will still use it, because they need the slow from rylais due to their lack of CC/ need of stickiness.
92% cost efficient
96%
since it means that the enemy is longer in his defile and that he has an easier time landing Qs, it might have not have been the best item on him but it was still a pretty decent item and Viktor always enjoyed it due to it allowing him to hit the second part of his laser and increase the overall DPS of his ult (people won't be able to escape it as easily)
where you build rabadon immediatly after morello since nothing is better,
building rylai