My opinion would be that 2003, 2008, 2013, and 2014 had the best casting. Despite the 2008 and 2014 season doing poorly, the casting was actually really good, and the issue was more so to do with what was going on behind the scenes.
I think in these seasons, the casting was very diverse and somewhat represented australia. Most of the housemates in these seasons were intelligent (mainly 2003/8), entertaining and they could hold interesting conversations. They also mixed really well and the relationships that formed were interesting.
IMO the casting in the years 2001, 2002, 2012, the people were too similar, and the casting was basically white washed. I didnt like the direction the show took from 2004-2007 being "hot, sexy, singles". IMO these housemates were not very interesting and all we saw of them was flirting and making jokes/having too much fun. No-one seemed as though they had depth or substance, apart from a few such as (Tim, Vesna, Dave).
In saying that... I do feel as though Australia is yet to see a season of Big Brother which accurately represents Australia and the true diversity we have here. I want a casting like TARA. I feel like past seasons have been polarizing, unrelateable and only representing a very small portion of Australia. I want interesting characters who can hold interesting conversation.. people with depth, substance. I want big characters who are going to make good TV, and also lots of light-hearted moments/not too serious.