Most likely convicts... pretty much just buy a male and a female for 4 or 5 bucks, and within three days they will spawn, unless you happen to find a pair that is totally unlike any other pair of convicts on the face of the earth... just keep them alone or in a tank full of other convicts, cause all hell will break loose when they lay eggs and whatever else is in there will be beaten or tormented to death... more fish may curb the aggression if there are enough of them and they are of a size where they can defend themselves against the onslaught.
Seriously, another good one and a little less of a danger zone would be yellow labidochromis. I have never had a colony of these that didn't breed and they are somewhat docile in comparison to the convicts...