The heartiest fish I've got (and have ever had) is a rainbow shark. It got it when it was about and inch and 1/2, when I first got a 5 gal aquarium and knew next to nothing about keeping fish. He's survived several tank swappings, countless water conditions, random stupid mistakes, overcrowding, lack of food, a bout of ich (and two treatment cycles), he even jumped out of a temporary (during tank upgrade) tank twice and flopped around on the ground for up to a minute each time. (none of this intentionally, just as my knowledge progressed)
Now he's in his own (large pot) cave, eating spinach, and his colors are more vibrant than ever. Truly a hearty fish.