I think this depends on how you define badass. I always think of a 'badass' as being more about how strong and resilient you are and less about the level of simple 'bad' or 'evil' you are. At the end of the day, and in the simplest terms, Gwen and Lilah are both BAMF, however, I'm going to buck the trend here and give my vote to Gwen.
Lilah is most certainly a wonderfully intelligent, conniving, beautiful and deliciously bad to the bones of evil character. She has proved time and time again that she can fight for right to respect in a man's world and isn't opposed to using anything she can to do so. I love her for not being a wishy washy character. Plus. She got to bone Wesley and THAT is something worthy of my ultimate respect 😛
I think Gwen has overcome so much adversity to end up making something for herself. Gwen can physically fight, a skill that she was not born with but had to learn. Gwen has complete control over herself and would never be owned by someone in the same way Lilah is. Gwen is independent and uses this independence to wager deals in which she comes out on top and I admire that skill. Gwen was born a freak but found a way to utilise this as a strength. Gwen uses everything that she can from the world around her to make herself successful, but does so at the expense of the wealthy, as opposed to at the expense of the poor.