Hardest fish: probably powder blue, moorish idol, mandarin dragonet and copperband butteryfly 
All of these are alive and doing fine in my tank but I still consider them some of the most difficult to keep, along with the achilles tang, never tried to keep it though. 
Easiest: My true perc, been alive for a couple years since i started the hobby 
		
	
		
		
		
		
		
	
		
		
	
	
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