Like
@Puppet said, obviously season 4, but it doesn't even have to be because of Weslah! Lilah never loses her integrity and independence during her relationship with Wesley, and it's interesting how she keeps working against him (like when she played him to get to Lorne, or all the times she tried to extract information from him).
Lilah is a real badass in season 4. She offs Linwood, makes Gavin her bitch, deals with The Beast, Connor and A.I. like a pro, survives on her own after the Beast trashes Wolfram&Hart even though she's injured, and still manages to find literature on The Beast that Wesley doesn't have. On top of all of this, she has some excellent lines and moments, especially with Wesley, but not just. ("Look, Angel, I know you've been out of loop for a while, but I'm still evil. I don't do errands unless they're... evil errands.")
I also appreciate how even though she's hurt by Wesley dumping her, she recovers and accepts the end of the relationship, as well as her eternal servitude, with grace. In Home, she's dead and still has feelings for Wes, but is 100% comfortable with herself and her life. It's hard to find villains who aren't either EVIL or trying to be good or liked, and Lilah is an excellent villain and character because she doesn't need to be explained or redeemed.