It's a tie between a melanachromatis auratis female and my current blue tetras.
The cichlid (I took as a rescue from a friend, not knowing anything about how aggressive they are when I was 14) ATE the eyes of 1 of my cory cats, and 1 eye of another, nipped their tail and fin to shreds, killed a lot of fish, injured the rest, all in less then 6 hours overnight. After treating and healing the community fish I rehomed them because no one would take the cichlid and I couldn't keep her in the bucket permantly. So she got the 29 gallon all to herself and costed my at least $200 in fish/meds. (I liked her personality though, after I forgave her for being a monster, she moved the gravel around in her mouth a lot, it was cute.)
The blue tetras I bought 8 for about $4 each, they got ich and while treating them in my QT/H tank, I used half the dosage of ich cure meds I got, watched for 2 hours and they were fine, left for the movie and got back 3 hours later, 3 were dead. Moved back to main tank (just heat treating it) and the next morning 3 more died. So now I have 2 tetras left, and I really don't want them but can't rehome cause they are my boyfriends favorite. So after everyone is well, I have to spend more money to get more of a fish I don't like.