The voice i dislike the most, maybe a surprise to some, but is cera's.
I would actually agree with this for LBT 5 and later in a lot of cases. For the softer singing parts I always found she did a pretty good job.
For the more aggressive singing parts ("The Mad Song" for example) and spoken parts, Cera's voice really doesn't work. Her voice really is well-suited to softer, gentler scenes, both when speaking and singing.
Ironic, as Cera is the most aggressive of the Gang of Five.

Anndie McAfee can do softer, mellow spoken parts very well as Cera, but her aggressive spoken words/singing just always seemed too forced and odd-sounding to me.