これ誰の？

this (indicating an item near the speaker, the action of the speaker, or the current topic); this person (usu. indicating someone in one's in-group); now; here; I (me); certainly

who

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question