If I were to answer this based on each season
2001: Obviously Anita, quite explosive compared to the rest of the cast who were more passive in arguments. I initially saw her as annoying when I first watched the seres as a child, however on a rewatch I actually felt sorry for her when she was pinned against Blair, Ben and Johnnie in a lot of confrontations. I credit Sara-Marie for being her only friend.
2002: Kieran (the original bareable one) was a great addition to the cast which I found terribly weak for a channel 10 series. I loved him the first time watched and I loved him on the rewatch potentially even more as I feel most of the 2002 season has aged horribly and is a giant slog to get through.
2003: Kim Marie was more entertaining than Jamie, but both were pretty weak in my opinion. I don't really have much to say about her as even though she did shake up the house slightly, I feel she probably hindered Reggie more at that time by allowing Reggie to feel closer with the two of them rather than the main house.
2004: The both sucked, I can't even differentiate between the two. Monica stayed longer, thats all I have to say. I for one did not find the Fitzy showmance interesting.
2005: Vesna (as I stated above). Racheal was an interesting character at the time, but really does she compare to Vesna? The 3 intruders that followed either fizzled out and became unlikeable or just weren't interesting.
2006: Perry is really the only one who I can remember for the most part.
2007: Zach was very likeable and fit in with the cast very well both on paper and in practise. The fact he came second says it all really, however the whole thing about him coming 2nd to Aleisha by the closest vote ever is all because of the constant showing of the voting results throughout the live show (same with 2008). At the beginning of the finale I believe Aleisha was winning something like 64% vs 36% ? Or something along those lines, either way not extremely close at all. So I would say he is over credited for that.
2008: I liked Cherry for his physical appearance lol, Terrance was fun I guess. I didn't find the intruders that season to be particularly strong.
2012: Look IMO they both really sucked. As far as Sam made it I did not find him attractive nor did I enjoy him as a character.
2013: Boog clearly was the stronger of the intruders, probably my third favourite behind Zach and Vesna.
2014: I have no comment other than I liked Penny and she came into my McDonalds when I was working a shift and I gave her a free coffee. I have no information to go off that is even worth talking about.
2020: I guess Sarah counts as intruder? Unless we consider the intruders to be the 4 that came in the crane? I'm not sure what the definition is for this series as all housemates entered within the first week. But IMO of all housemates to not enter on Day 1 Sarah was the superior character and player, who I think was very likeable and I enjoyed seeing a younger housemate outside of the typical Skye Wheatley archetype that we've seen casted in previous series.
So really the only contenders here are Vesna and Zach, maybe even Boog. I think the fact Vesna came earlier gave her an advantage over Zach, but I just think Vesna made better television and is easily one of my favourite housemates of all time, and probably even my #1. She also holds the record of spending the most days in the Big Brother house as an intruder with a total of 75 days. Clearly the voting audience at the time saw what I see watching it 15 years later, so I cannot find a reason to not consider her the best intruder. Mainly also because she fits the criteria; she brought more to the table than most of the cast did who entered on day 1.