The livebearers are by far the easiest to breed (as above; guppies, platies, mollies, swords etc) and its easy to tell the males and females apart. The females are pretty much always pregnant (even if only kept with a male for a short period) and give birth to fairly independent live young.
Convict cichlids, kribensis cihlids, live-bearers, danios (if you can separate the parents fast enough), aenas corydoras (again, separating parents from eggs afterward).