Took the words right out of my mouth. Hating either of the girls would just ruin the show. I started out not liking Sheryl at all, but eventually I grew to like her about as much as Ranka (though I still don't like what the creators of the show did with her hair
). On the actual singing side of things I like Ranka a lot better. Just listen to Seryl Aimo then the original Aimo and it's clear who is the winner. I couldn't actually listen through the Sheryl version all the way.
I also prefer Ranka's singing. Though I really enjoy Sheryl's voice, I generally only listen to her in the duets. The only song of hers I listen to on her own is Diamond Crevasse. You're right about the Aimo, though. Sheryl's rendition was absolutely terrible. Ranka transitions to Sheryl's songs much better than the reciprocal.
If we're talking about Macross songs, my top favorite is What 'bout my Star?@Formo with Ranka and Sheryl followed by Watashi no Kare wa Pilot by Ranka.
My favorite is Lion, Northern Cross, Triangler, What 'bout My Star? (Sheryl on Stage), Welcome to My FanClub's Night! (Sheryl on Stage), and Infinity which are all by Sheryl only. The Nyan Nyan Special Service Medley (Tokumori) is with Sheryl and Ranka. No, I'm not being biased. Ranka's only good song is actually in the Nyan Nyan special service. If you listen to the Nyan Nyan song going 9:01 into it, that's like the epic favorite part!
I don't think you understand the definition of biased lol.
I still can't stop singing that darned 'Ninjin Love's you yeah' song. It's only like one minute long, but the music is so catchy. I also really like Ai Oboetemasuka, Aimo, Anata no Oto, Watashi no Kare wa Pilot, and that quick little Chinese restaurant music. I liked What 'bout my star a lot when I heard it on Kawaii-radio about 4 months ago, but I sort of played it too much at the time and killed it. On the Sheryl side I enjoy Diamond Crevasse, Northern Cross, and Lion. Best two songs on the whole album, SMS Shoutai no Uta (I imagine some day with my anime friends getting drunk, swinging a pint of ale around and singing this song, "Susume, susume esuemuesu!

" lol) and of course the Triangler duet.
Oh, and 'Tally Ho' is really awesome as BG music. A few others from the BG I like, but I can't remember the names.