Its yoona coz:
1. SeoHyun is the closest to Yoona, even before their debut.
2. SeoHyun's house and Yoon'a house are quite near
3. Yoona is SeoHyun's sunbae in her Senior High School.
seohyun is closer to yoona because:
1. they were childhood friends even before debut.
2. their houses r near each other so they can talk and play.
3. yoona is the second youngest which means she can relate to seohyun a bit more than the others can.
4. Yoona is the one who is the nicest to seohyun.
5. Seohyun gets more influenced by yoona than the others since yoona and seohyun talk more with each other.
For me, its Yoona eonnie, though Hyoyeon eonnie is currently the roommate of SeoHyunnie, we know for a fact that the two are known as childhood friends and they are almost same age. for that, i can conclude that they share common interest and hobby.
Seohyun is closer to YoonA. Yoona, the type of person who is not competitive and Seohyun who is calm and peaceful make the perfect match. But YoonA and Yuri are the closest.
