Taeyeon is the leader. :) I personally think all the girls in SNSD would make awesome leaders, but Taeyeon is the best when it comes to leadership skills!
i wish it was jessica but its taeyeon. But i love taeyeon and she is leader of SNSD 4ever. if anything happened to her Sica would take charge has she is the 2nd oldest in SNSD. Taesica rules!!
Taeyeon is the leader of Girls' Generation. I think it's because she's the oldest member, but Jessica is the second oldest member by a month. Taeyeon is known was the 'kid leader' because of her small build, but according to Girls' Generation, Tiffany (Manager Hwang!) has the most leadership followed by Seohyun and Hyoyeon.
Yoona is definitely NOT the leader, although she did gain a lot of attention for her group because of acting in the drama 'You Are My Destiny'. She's also the visual/face of Girls' Generation, meaning she represents the group in receiving awards (and stands in the middle of photos because she's pretty) etc.