Close between BB4 and BB16 but I went with BB16 because at least BB4 is memorable as 'the boring one' if that makes sense, so it's memorable for something
And it aired when BB was more relevant so it's kind of remembered by default. Whereas because BB16 aired at a time when most people don't care about BB anymore, it won't even be remembered by the general audience as the boring series, it will just be forgotten completely.
And about BB2, I think there's loads of memorable moments personally (but I'm a massive fan of Brian so that's probably why there is for me since he
was BB2).
Edit: I haven't seen BB12