According to Herodotus “Isis … is Demeter in the Greek language”. [Herodotus Histories 2.59] According to Diodorus Siculus: “Osiris when translated is Dionysus, and Isis is more similar to Demeter than to any other goddess”. [I 13.5]
Many modern scholars have used these texts to identify Isis with Demeter. Both goddess have an association with agriculture. Terracottas of Isis Thermouthis, depict the goddess with a torch, a symbol of Demeter. Yet the historical reality is more complicated.
Firstly Isis could also be identified with other goddesses. The most obvious is Aphrodite (and even Astarte or Isthar). Several terracottas have been found sharing attributes of both Isis and Aphrodite. The Roman love poets clearly understood Isis as a goddess of love.
In some ancient texts from the Greco-Roman period, Isis is associated with all the gods including Deo, Rheia and Hestia. Isis is transcendent of them all.
At the same time, we often see both Isis and Demeter named in the same inscriptions as separate goddesses in Egypt. This suggests that they were understood as separate divinities.
Secondly, Isis was worshipped across the Greek and Roman world. She kept many of her Egyptian symbols and attributes. There is some evidence that Nile flood festivals were celebrated outside of Egypt.
Outside of Egypt, Isis was understood as her own person, not as an Egyptian version of Demeter.
Thirdly, even with the Egyptian tradition Isis was associated with other goddesses like Hathor, Renenutet and Beset (the partner of the god Bes).
The answer to which Greek goddess is Isis like is complex? If you are searching this for a pop quiz, mid term or pub quiz then the answer you want is Demeter. But if you want to be clever, the question could be answered by saying all goddesses or none.