Bit of a tough one this, as I really can't dance to save my life but would love to be able to.
I guess I'd have to opt for either Lilia or Karen, as they both seem to have done wonders in the teaching deaprtment with Darren & Bill.
Thinking back to week one & the judges comments about Darren's hands (i.e. the rhino remarks), Lilia seems to have brought him on to the point where he actually looks to be a real contender for the title. I don't believe this is simply down to the fact that Darren must have had some hidden talent; after all, the same hing happened with Aled, last series, but is a case of a good teacher being able to encourage & develop upon his latent ability & his growing sense of confidence & enjoyment in himself & the whole SCD experience.
As for Karen, she seems to have been able to bring out an abiltiy to develop a story, in Bill, with their routines that few of us would have thought was there; not a criticism of Bill, BTW, just a case of the fairly straight laced image that alll newsreaders tend to develop as a part of their job. Add to this the fact that she has even managed to get him past the hurdle of being very nervous & uncomfortable with the close physical contact the tango required, whilst still allowing some very entertaining choreography; always something a little different that makes them stand out from the rest of the competitors, & it all indicates that she's not just a great dancer but is able to pass on this ability to others.