Which Adventurer Ring Is Best AFTER Big Bang Updates?

If crit class post Big Bang, Critical ring is the way to go. If not, unless the bonus crit damage from the magical ring also got removed it will be that, or if it was removed then Critcal ring.

Crit ring: +10% crit chance, 2 attack, insta death bla bla
Cruel ring: +5% crit chance, 3 attack, ^
Magic ring: +5% crit chance, 1 Attack, stun bla bla

Everyone crits after Big Bang. 5% more crit chance = 5% chance to do at least 135% damage on average = 1.75% extra damage even without any crit skills. One Weapon Attack is not going to be more than 1.75% extra damage except at very low levels. You've got to have less than 57 total Weapon Attack and no crit boosting skills or equips for Cruel Ring to be better.