Have not seen every series and will update this post when I've watched more.
Will not do every series as I feel there are certain ones were housemates exits have truly impacted the series.
Big Brother 2 - Narinder
Everyone that watches BB2 nowadays will definitely notice a decline in quality post Narinder's eviction and despite some funny quotes from Brian and the Helen and Paul love story that ish kept the series alive in some proportions. It's such a shame because the first half of this series is so good that losing Narinder would definitely lead to tragedy.
Big Brother 4 - Federico and Jon
I know many will always say Anouska should never have been evicted and her eviction killed the series but we never got to see enough of her to really know what she had to offer long-term, sure she was really nice, very flirtatious and seemed like one to speak her mind but in the end it has to be Federico and Jon's double eviction that impacted the series in the worst way. They were the only two characters left in that house that were actual good housemates (before lisa came in). With Jon we were losing the protagonist/ the big talking point of the show and Federico, the windup merchant and the antagonist (in a way). So with those two gone any chance of a conflict heavy/entertaining house was down the drain, thank goodness we got two weeks of our Lisa
Big Brother 6 - Maxwell
For the better!!! A lot of series can be known to have a house divide that can be very defining to the series. What I loved about the English vs Ethnics divide was that it was quite short but sweet and made way for the second-half to bring some new storylines i.e Pregancy scandal, Kemal's paranoia, Craig and Anthony, Orlaith wanting to walk, Science against everyone, and Kinga's epic return. Nevertheless English vs Ethnics has to be my all time favourite story line of all time
Big Brother 7 - Grace
Very difficult one, has to be either Nikki or Grace but I would have to say Grace mainly due to the fact that very early on we lost the series antagonist, Her downfall was quite lousy in retrospect would have been amazing to see her have a true downfall instead of being evicted from someone that was pretty much in theory part of the audience of the baying mob public that were screaming
'get grace out' It is definitly arguable to say Nikki's original eviction hurt the series but post Nikki I found to be great fun and really enjoyable and found that I could get to know the rest of the housemates a lot more without it being the Nikki show. (Dont get me wrong I love Nikki!) but there were still some fun moments like Automated Big Brother, Prison Task
Big Brother 8 - Charley
Not very surprising as Charley was the true saviour in a series that was lacking in good characters so her eviction clearly sealed the fate of the last 5 weeks were we were introduced to 4 out of 5 very forgettable halfway housemates to make up for the massive loss and Carole's everlasting iron fist reign over the house (is that a plus or a minus) Despite guru week seeing the fall of the Greek freak post Charley is not good.
Big Brother 18 - Lotan
Surpringly a housemate I have a lot of hate towards exit proved to be a negative rather than a positive for the series. The Rose Cottage/Thorn Cottage by Week 3 had reached boiling point as new resident Isabelle was introduced and the tensions in the house were intensifying by the day eventually leading into the infamous fight night 3 that saw Lotan chucked out of the house. Although a satisfying end to this scumbag, leaves me often questioning at the same time, that the series may still have been in need of Lotan moving forward a once we was ejected the divide became no more and what was introduced to us a second half of pretty inconsistent episodes. I don't think BB18 knew what it really was in that second half tbh. I still really enjoyed the series tho!
Will Update this once more series are complete