Definitley Lex. Lionel Luther is also a good character. Without them I don't think the show has the same spunk. Like when Lex and Clark weren't friends at the begining of this season, the show just wasn't the same. Lex brings a lot to the table that the other characters just can't.
Believe it or not, Lex is my favorite Smallville character - he's the most well-rounded (other than Clark) and interesting of the whole cast - and with Lex around, you can be sure things are never dull!
I like Clark and Chloe. It is very hard for me to choose between the two of them. I think they are both great at acting, and I love the character they play. It is very interesting to watch them.