Erm, there is too many to chose from!
Soooo i can''t just chose one 
I do like the kiss scene in 1.5, kind of but it is a bit to corny for my liking and Claudia's expression afterwards really annoys me - i don't know why.
I love the moments before and during Stephens death scene from the 3 way arguement up until Cutter on his knees - it is just played so well and with a lot of power.
I love the moment from 2.6 when Helen wakes Cutter up in the red room thing with a kiss - it just says so much about those two characters in such a short scene thanks to both of their brillant acting.
Last but not least i tend to favour all of Cutter/ Jenny's banter/arguing scenes because some make me laugh and others can be very powerful, and yes they do argue like an old couple.