The Cat gets better with each series
Perhaps 'A Fishing holiday??' in series 4 was where the Cat started to truly shine. Series 5 was so heavily tilted toward Rimmer focussed storylines that it's hard to say he truly shone compared to the others but we did see the introduction of Dwayne Dibbley. In series 6, after the departure of Holly, he got some more lines and was frequently great- but, again, the series was of such a high quality that it's hard to point him as being a standout.
But in series 7 he had the great 'Talked about cooped' part in Duct Soup.
Series 8 was an experiment in a way but not one that did the Cat (or Kochanski) many favours.
But, since then, the Cat has been an increasingly stylish genius of deadpan comedy. Such as the coincidence virus/injection. One character says something and the Cat's expression remains stony, only quietly flickering as you know he is supressing something. He also more often has lines that are less wacky, more dryly cynical. I suppose it was always there through series 5 and 6 but he has really come in to his own style.