Dojo loaches. I have never had an issue with any other types of loach, but I cannot keep dojos alive long term. Also, platies. I buy them, I like them, they drop dead. I buy them, I like them, they drop dead with no warning at all. Though, I believe the biggest issue with platies is actually finding decently bred ones and not mass produced junk.
Two roads diverged in a yellow wood, but thankfully they both led to fish stores.