The bright blue micro sized zoas. Haven't tried the other colors, and you rarely see them for sale anymore. Used to go by the names Blue Microdot and Bahama Blues. I never had trouble with Arms, PEs or caribbean ones when I ran really high flow and light, but nothing would make the Bahama Blues happy.
I would have to say the hardest one for me would be my darth mauls. i can keep them but do not get good growth from them maybe 1 to two polyps every two months and the colors seem to fade as they get older and I have tried them under 15k 18k and numerous t5 combos.. but my harder ones utter chaos/ cherrie charms seem to do fine.
Interesting responses so far. I haven't had a Zoa so far that I could not grow out! Knock on wood! Dearth mauls included. Even under halides top shelf of tank. Interesting to hear from all. I credit my 'dirty, warm water'.
Made a poll for this a while back. I think the consensus was that darth maul were the worst offenders. Not necessarily the most prone to melt, but rather the greatest number of people had them melt. Probably because they are more abundantly available than say, soprano, for instance.