I would say the worst fishes my husband bought were the RTC who ate up everything and tried eating up my PNT's tail, which was the final straw....I gave it away.
The other was the trio of Asian Climbing Perches who were so aggressive at feeding time they ate up all the pellets before my Flame Pike Cichlid could get to it. Those were given away too.
I guess a worst purchase on my end would be my Gulper cats though I wouldn't consider them a bad purchase even though I have already sacrificed a few expensive fishes down their gut.