Popular by a country mile. Mainly because it allowed us to see what the girls could do when not constrained by slaggy/vacuous pop nonsense (omg, I've turned into Sheila...

)
I Could Have Danced All Night was pretty damn storming too, and I was never a Lauren fan (see the mighty monkseal's comments on line-by-line acting for a better explanation of why I didn't love her than I could ever come up with myself).
For 100% commitment, I liked Jessica's Cabaret; for actually getting the tone of the song right, I think Sophie's Gonna Wash That Man was overlooked - it was one of the performances of the series but had the bad luck to be right before THE performance, combined with Steph taking her direction a bit too seriously; Mambo - the first time - was a blessed relief as I thought people finally had a chance to see what I could see in Danielle.
I don't think it was a classic, but Jenny did a lot better with the Clowns than she could have been expected to do. And Bronte was pretty much spot on with what she was given - the only real 'shock' result, IMHO. Erm... who am I forgetting?? I know there's someone...
There's one other truly, outstandingly memorable performance that I've you-tubed quite a bit, but it's for all the wrong reasons and I don't want to be bitchy on final night so I won't single out the girl involved... No, sorry, I am a bitch, I can't resist saying that I did NOT have fun.