Malawi cichlids have been the absolute toughest and hardiest fish I've every kept, but I suppose I don't really know because I try not to test the hardiness of my fish if you know what I mean.
The hardiest tropical fish I started out with were Zebra Danios. I've used Zebra Danios whenever I cycle a new tank or simply kept a school of six by themselves or with other tank mates compatible with them. Keeping Zebra Danios has never been a disappointment in my years of fish-keeping.