His April Fool's joke (despite it not being April) about Norweb and Lister's bank account is one of my all-time favourite moments. As is his chess battle with Queeg. And the dog's milk discussion:

Holly: "Nothing wrong with dog's milk. Lasts longer than any other type of milk, dog's milk."
Lister: "How come?"
Holly: "No bugger'll drink it."

Red Dwarf was so good. Just a pity series' 7 and 8 were pretty bad. I haven't seen any of those episodes since they were aired.