Yukirin (Kashiwagi Yuki) but, she's leaving the group. The runner up is Yuiyui (Oguri Yui), the girl in stripped dress and black cap at 0:37. Other outstanding ones popular wise are Yuiri (Murayama Yuiri - the girl with black jacket at 0:44) and Naru (Kurano Narumi - the short girl in stripped hat at 0:37)
Yukirin’s level of popularity is waaaay to high compared to the other members.
The only one to come close to her is Honda Hitomi but she already left and is in hiatus.
Members wise Yuiri is very popular IN the fandom not so much with the general public. She doesn’t have the visuals but fans love her for her personality and determination, it is basically her loyal fanbase that got her to the front row.
Yuiyui is the current face of AKB, I like her a lot, great center.
Narumi is somehow popular but niche, she is the current GM and is alright I guess.