The Mane Six are all fantastic singers... But they need something that suits who they are...
AJ should sing something like T-Swift. Rarity's Everypony Should Know did that Pinkie needs to sing Katy Perry's "Last Friday Night" though she did wonderfully with Smile Smile Smile RD should sing Pink Fluttershy is best suited for singing something older... like "The Sign" by Ace of Base And Twilight? Twilightlicious was a pretty good song too.
My favorite singer would have to be a tie between Rainbow Dash for her solo in "At the Gala" and Pinkie Pie for "Smile, Smile, Smile". That said, the best singer, for me, would be either Rarity or Sweetie Belle. Guess awesome singing runs in the family.
Brothers: NEVER Scoot: cute voice, would vote if the lyrics of that song were better Twilight, Applejack, Rarity: Meh... Don't fancy those songs. Voice actors still amazing. Especially Rarity's Sweatie: She IS good. Once sang her "hush now quiet now..." in a full 10-passenger gondola lift with one brony I met in the same ski resort. Believe it or not. Dash: Would be absolute number one if she had some official/at least good metal song Applebloom: Does she sing? Fluttershy: Could vote her coz she's Fluttershy. Awesome soft voice And yet, I MUST vote Pinkie, you know why.
I'm going to vote Pinkie since she seems to sing the most although I would prefer to never hear the song she did in season one episode 2. I haven't had a chance to hear Sweetie Belle in the hearts and Hooves day episode so I might change my mind after I see that.