I've seen 5 (well, all but the finale atm), 6, 9, 10 and 11. I'd strongly recommend 5, it's been amazing imo and one of my favourite series worldwide.
And it feels a lot less sanitized than the Channel 9 ones because of the Uncut companion show (which was sometimes messy because the BB5 housemates were horny as **** and a lot of it is just smut you can skip if I'm honest lmao) but was also crucial in getting the full picture on certain controversial events in the house so having that definitely added to the series I think.
I really enjoyed 6 too but watching 5 straight after and loving it so much has sort of overshadowed it a bit in my memory.
And of the Channel 9 ones of course 10 was my favourite, my mind's fuzzy on 9 but I don't remember it as anything special and 11 was pretty meh.