When Sapphire breaks out of the Darkness's possession and falls into her partner's arms. Made even more powerful and poignant by featuring Steel's one and only smile in the entire series.
Big Finish Audios
Silver's interactions ( and concern for) Mary in All Fall Down are positively adorable.
In Water Like A Stone, Ruby, after acting rather stand-offish and cold towards her for most of the episode, absolutely freaks when she believes that Sapphire is dead.
Likewise, Ruby's rescue of Sapphire and Steel in Second Sight and her hastily concealed relief at seeing them back in action. Also, her interactions with the other Sapphire and Steel towards the end, and her remark about how she thinks they might actually work quite well.