I guess only consider actual skill and technique, and not how nice her voice is when singing.
I think most of us are too n00b to actually recognize skill and technique. Besides, how her voice sounds is probably more important than technique... I mean, ideally you want both, but if someone has epic technique but still sounds like a chainsaw then I wouldn't want to listen to her.
Anyway, if I were to select a senbatsu sized team of the best singers in AKB48 only, I'd pick:
Iwasa Misaki
Katayama Haruka
Kojima Haruna
Maeda Atsuko
Nakaya Sayaka
Akimoto Sayaka
Kasai Tomomi
Kashiwagi Yuki
Masuda Yuka
Sato Amina
Sato Natsuki
Iwata Karen
Acchan may be a personal bias, but I found her singing on her solo single very soothing.
I didn't list Takahashi Minami because she sounds way too nasally for my taste. She does add some power to AKB48's sounds, which is great, but I wouldn't want a solo single. Since we're talking about technique as well, I noticed Takamina doesn't seem to have that much stamina. She sounds out of breath after one song when she sings live, which makes me wonder about her singing technique. Of course Takamina does have her moments, I've seen perfs of Bird and Itoshisa no Accel which were very nice. She's good, but not top 12 worthy imo.