In my opinion, the Y plate and 51 plate ones are the best!!
They were put into production and had NO cost cutting at all.
This means a multitude of things, such as carbon effect trim in the doors, which looks nice, better fabrics, etc.
Also, the early Z's and that includes for ZS and ZT, seemingly had more robust power units. Around 6 months thereafter, the cars started to appear that had oil consumption problems. I don't know what happened as I was not a powertrain engineer, but I would have a good bet that they tried to cut costs on the power unit somewhere as there were a lot of cars on mid to late 51 plates and after that ate oil and knackered bottom ends, etc. . . .
If I were gonna buy a ZS180, I may well hunt down a nice early example which seems nice, with a quiet engine, etc. . . .