Least Favorite character?
Logan, I think. He was just so wannabe :P And I also hated Sean Faris' character Ben, though I love and adore Sean Faris as an actor. His role was just not explored enough.
And I have to mention 'ripper' Stefan. I like him being more like a real vampire, which is what the writers call a 'ripper', but the season 1 'ripper' Stefan was just a poor attempt to steal from the greater vampires in older vampire fiction.
Of regulars, Bonnie and "Elena"(she isn't really important, but she is supposed to be). Of randoms probably Logan or Kayla Ewell.
Elena, Matt, Vicki, Andie, Bonnie, Jeremy. In exactly that order.
Matt..then Vickie and Logan.
The Cardboard King and Jeremy.