My favorite singers would have to be Lady Gaga, Lorde, Lily Allen, and Mary Elizabeth McGlynn, with Lily Allen being my absolute favorite, followed by either Lady Gaga or Lorde, and then Mary Elizabeth McGlynn. As for groups, I don't listen to groups, unless a band is considered a group, in that case there are 2 groups/bands that I do listen to every once in a while, Queens of the Stone Age and the Brazilian band CSS/Cansei de Ser Sexy/Tired of Being Sexy.
As for my favorite songs, it's a long, long list. For Lily Allen it would definitely have to be "**** You" or "The Fear", but I listen to "Smile" on a more regular basis due to it's more kick backed tone. For Lady Gaga, it would be either "Poker Face", "Bad Romance", or "The Edge of Glory". For Lorde, it would be either "Team", "Yellow Flicker Beat", or "Tennis Courts". I also listen to "We Can't Stop" by Miley Cyrus on a daily basis, almost the same thing goes for "TiK ToK", "We R Who We R", "Take It Off", "Your Love is My Drug", and "Crazy Kids" by Ke$ha (Just going to get this out of the way and say that I have giant weakness for songs about partying or have electronic sounds.), then there's "You and Me" from Polar Bear's Cafe, "Thrift Shop" by Macklemore and Ryan Lewis featuring Wanz, "Alone" by Armin van Buuren featuring Lauren Evans, and very recently "Chandelier" by Sia.