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Seohyun never makes a mistake when she dances and she has good technique and control. If she has a flaw it's that she isn't very passionate when she dances.

Jessica has natural skill for dancing but is a lazy dancer. She was actually almost as good as Yuri and Yoona and better than Sooyoung in their debut days. But she's not living up to her full potential.

Sunny may have a love for dancing but she messes up a lot and can't pull of all kinds of dancing. If she was sent to a dance battle with Hyoyeon, Yuri, Yoona, and Sooyoung she wouldn't be able to keep up at all.

Taeyeon's dancing is good but there's nothing special about it.

Tiffany is a very hard worker but not a good dancer. She's the opposite of Jessica lol.

This video isn't just of Seohyun but it proves my point pretty well:
link

^ she must be really good at choreography but was she sick or something cause her face was very blank. On that note so Seohyun is a good dancer because of skill or she has never makes mistakes?