Yuka is the number one. Best proof is Higurashi no Koi, I think. She does a well job singing Mushi no Ballad, too, although that song was made for Sayaka voice only. Yuka deserves a solo song in a future stage.
Sayaka (and I'm not being subjective on this, this is one of the reasons why I chose her as my
oshimen) is another amazing voice in AKB, she doesn't need technology help to prove it. It's strong (sometimes I feel she doesn't even need a mic ^^;), mature and low pitched. Kimi wa Pegasus, Mushi no Ballad and the shuffle of Itoshiki no Natasha are some of the proofs.
Takamina also gets my vote. As maliciel said, her voice is a bit nasal, but not as nasal as Haruna's
Best proofs of Taka's voice? The intro of A5's Nagai Hikari, Bird and Juunai no Crescendo. I didn't really like her Mushi, though. The singing was good but it lacked power.
Finally,
Nonti. Listen to Kimi wa Pegasus, too.