I respect Mariah a lot, but I think she's in third place.
In cultural impact, I'm gonna be real. Oops has a lot of cultural impact and probably has the edge at the end, but 21 had a lot of impact. The Grammy night domination was insane, the BRITs performance will probably be timeless, Rolling In The Deep will probably be a break-up anthem... Oops probably still wins in this category, but not as much as a lot of people assume in my opinion.
21's commercial success is surreal, when you think about it. The Fame (Monster) is the closest thing we have to an Oops-sized era, considering album sales and whatnot. 21 has nearly matched its sales in less than 2 years over a decade later, with the digital age and piracy being an infamous album sale killer. 21 crushes both in this category.
The cultural impact one is the one making me reluctant to answer. We haven't truly seen the impact of Adele's 21, as its still selling and fresh in our minds. But honestly, it's closer than I initially thought (I thought surely that it was Oops).