The British Museum, the biggest collection of stolen goods in the world.
I'm also torn, because one the one hand the plundered items the British Museum and other UK museums hold should be returned. On the other hand, being outside locations where they might be destroyed[0] e.g. Iraq,Syria, and also the sheer budget and technique to preserve things are both arguments to keep the collection the way it is. The preservation vs plunder is a long running argument[1].
Modern techniques enable digitizing[2], but that wasn't available previously. Digitizing is a path forward that's less controversial (though still isn't quite the same).
Then there is the Kohinoor[3]. Let's chalk that one under colonial plunder.
[0]: https://www.newscientist.com/article/mg22530125-000-can-we-s...
[1]: https://merip.org/2018/05/preservation-or-plunder-the-isis-f...
[2]: https://www.thedailybeast.com/can-we-digitize-history-before...
[3]: https://www.abc.net.au/news/2022-09-17/indians-want-kohinoor...
There is some benefit to spreading historical artifacts around to protect them over the long term (i.e. we can't know what any place will look like in 1000 years), but only if there are multiples, and it still has to be given not taken.
I did say that I wasn't clear that this had been plundered (as opposed to purchased), but I'm not sure that matters either. It's a piece of cultural heritage and the British were unwelcome guests.
Similarly, the British say the Kohinoor[0] was transferred legally (surrendered). But if is made under duress and it doesn't seem valid.
I think preservation vs plunder argument is an easy copout for those that are keeping stolen goods.
Even if you accept this premise, there are a good handful of other countries relatively untouched by war/instability that would be safer homes. The argument is a thin fig leaf.