BB2 is quite a forgettable series actually. There isn't really any stand-out or big moments that are particularly memorable. It being in between BB1 - the first ever and original and BB3 - the most popular and highest rated series doesn't really help it either tbh.
BB4 and BB12 are up there too. BB15 as awful as it was, isn't really a forgettable series. If anything it's probably one of the most (if not THE MOST) discussed series on this forum. Admittedly for wrong reasons but still. I mean whenever someone mentions BB15 or a certain housemate from that series it soon turns into a five-page discussion about how awful the production was or how awful the housemates were.