Interrogative pronoun used when choosing from three or more items. For two items, use どちら or どっち.
Which one do you like best?
この中からどれを選びますか。
Which one will you choose from these?
Interjection used to prompt a decision, express hesitation, or encourage action. Often used when the speaker is thinking or urging someone.
どれ、やってみようか。
Well, shall I give it a try?
どれどれ、見せてごらん。
Now, now, let me have a look.
Which one is your bag?
Well then, shall we get started?
There are so many things on the menu, I can't decide which to have.
Which of these is the tastiest?
何れ is the interrogative form used to ask 'which one?', whereas それ refers to a specific thing already identified.