Without question Mark and Karen's Salsa. For two reasons:-
1.) It's Mark and Karen
2.) And if that was not enough then, in the VT preceeding the dance Karen stated that the Salsa was the one dance that shouldn't be over-disciplined and that it was more about two people having a great time and their obvious enjoyment filtering through to those around them. And undoubtedly, the routine, music and atmosphere generated by their Salsa achieved this in fantastic measurel. As a non-dancer, I just found it toe-tappingly infectious.
If they reach the final, this dance will probably be the one most people will be looking forward to. And if they do as well, they will score the 10's the dance richly deserves.
Now there will be pedantics amongst you, who will argue that I'm not being very objective and that this competition should primarily be about dance, technique etc and all that garbage. You are of course entitled to your point of view, but I would argue you talking inane drivel. The vast majority of viewers will be non-dancers and as such will base their preferences on a whole multitude of subjective criteria many of which will have little to do with dance.
Also, on my list of top dances:-
Mark and Karen's Paso and Tango
Matt and Lilia's Waltz
Peter and Erin's Samba (Yes, Peter was crap and fantastic in equal measure)
Loiuse and Vincent's Jive