depends on what size, I've seen 3" convicts stand up to and challenge 8" jaguars. The jaguar backed off. I think as far as the meanest goes you'd have to consider size, like as far as an 8"+ is conscerned jags, dovii, devils, n a few others fit the bill, but cons at 5" and under are brutal especially during breeding. The african kenyi is also pretty mean, a friend of mine has a 4" kenyi completely dominating his 55 g. african tank of at least 10 various african cichlids.