When Ben transforms into Alien X he has to get two of Alien X's inner voices to agree with him to do anything, one is Serena. Serena is the voice of love and compassion, the nicer and sweeter one between her and Bellicus. Although she is the more compassionate one she is just as stubborn as Bellicus and thus the two have argued for centuries.
I felt all the actresses did well, giving Serena a sweet and caring voice.
I will say that, even though I liked they went the extra to get her, Vicki seems to be trying too hard to sound sweet and doesn't come off as natural as the other two. Plus she's too melodramatic in her acting, it sounds rather fake.
I do like both Kim and Tara but between the two of them I like Tara's more playful quality in her voice which I felt gave her more personality.
I say Tara is my favorite one here with Kimberly as my second favorite.