Potentially BIGBANG they always have crazy hype but longevity depends on the song. But they're now in their 5th year of hiatus so who knows whether the public will be receptive of their comeback or not after all the scandals. GD alone isn't strong enough.
afaik bigbang has never released at the same time as IU and twice for sure have not bested IU, none has.
The one time GD & tayeang released a song at the same day (from the same show), IU song set UL record and PAK hour record that stood for 3-4 years before they changed the charts (ul record broken by herself)
She holds all the chart records bascially when it comes to release dominance.
And none is even close when it comes to total streams and downloads (gaon certified)
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I remember this.
IIRC HwangTaeJi actually topped the first hour on Melon, closely followed by IU x Park Myung Soo in second place and Zion T x Haha third (all three were on the roof). But then IU x PMS caught up and overtook.
HTJ also had the disadvantage of performing first and being partnered up with Kwanghee who was the newest and least popular cast member, whereas IU had PMS who always found the most chart success from the song festival. That's not to say IU wouldn't have overtaken and smashed records had she partnered with someone else, but she definitely had the upper hand here. So I wouldn't say this is the best comparison.
The only way we would know is a full on head to head (my money would be on IU slightly edging out BB) but sadly I don't think we would see this happen with how far and few between BB releases are.