Ah Smallville... my guilty pleasure.
Favorite Character: Lionel Luthor without a doubt. I adore John Glover and he brought such life to the character of Lionel. I was a weepy mess when I realized that his character was actually, truly dead.
My heart would break every time the show would go back to Lex's early years and reveal a bit more about the horrible things Lionel did that shaped Lex into the man he became. More than a few times I cried tears over the injustices that Lex went through as a child and mourned for 'Alexander', knowing that side of him would eventually be silenced forever.
Least Favorite Character: Definitely Lana Lang. Ick. Phew. Bleargh! I hate Lana... hate hate hate her! Glad she's gone, dreading the fact that she will be back for half the season this year, but hopeful that something absolutely horrible befalls the character.
Favorite Episode: This is a rough one for me... there are so many that I love! Any of Lex-centric episodes where the inner struggle between the 'good' Lex and the 'bad' Lex are at odds. Those rare glimpses where, just for a moment you start to believe that maybe there is hope for Lex after all... if only... and then you remember that no matter what the writers on the show do, the character of Lex Luthor is a long established member of the DC Universe and is purely evil through and through. But Michael Rosenbaum's acting brings such feeling to the role that I can't help but feel like I've had the wind knocked out of me when 'bad' Lex wins out over 'Alexander'.
Other favorite episodes: Too numerous to mention although I was not a huge fan of the first couple of seasons... The whole 'Meteor Freak of the Week' thing got really old.
As for Tom Welling, yes ladies, I'm not blind... I recognize pure hotness when it graces my television,

but I'm a Luthor Gal I guess. I personally don't see how this final season is going to be all that watchable without Lionel or Lex in it, but I'm going to tough it out. Especially knowing that Kara is gone.
I'm also a big enough Comic Book dork that I'm looking forward to much JLA action this season. Hopefully that will make up for my missing Luthors.
I keep a close watch on Kryptonsite to read the latest news and casting information on the hopes that MR will finally agree to come back for at least a few episodes. /sigh