Which (one) — an interrogative pronoun used to ask which among three or more options.
この3つの中でどれが一番好きですか？
Which of these three do you like best?
どれを買えばいいか迷っている。
I'm wondering which one I should buy.
新しい携帯、どれにする？
Which new phone will you choose?
An interjection used to attract attention or start doing something — like 'well then' or 'now then'.
どれ、始めようか。
Well then, shall we begin?
どれ、見せてごらん。
Now then, show me.