Which dinosaur was the rarest?

Several dinosaurs are extremely rare in the fossil record, so the list would have to be very long. However, one of the most intriguing of the rare fossils must be Deinocheirus; this animal is only known from a single pair of arms which were discovered in Late Cretaceous rocks in the Gobi Desert of Mongolia. The arms are truly enormous measuring over 2 metres in length. The puzzling thing is that no more of such a large creature has been found. It is therefore not known how the animal might have looked in life.