It's a boring, obvious answer but Jim and Denise in the bathroom when she's going into labour in the Christmas special was just sublime. Brilliant lines, like when Denise's waters break and Jim says tenderly "Are you sure it wasn't just a great big piss love?"
The music on Denise's birthing tape was a master stroke, Ave Maria made that scene so incredibly moving, you could see Ricky's tears were real. And yet despite the emotion, it was still brilliantly-funny.
Hand on heart, The Royle Family is my favourite comedy of all time. It has everything for me and I doubt it'll ever be bettered. I think maybe it helps if you were brought up in that very working-class "poor" environment and get all the cultural references.