Iwi: This is the most common and general term for a tribe, referring to a group of people with a shared ancestry, history, language, and territory.
Hapū: This is a sub-tribe, a smaller group within an iwi.
Whānau: This refers to a family group, but can also be used more broadly to refer to a close-knit community, sometimes with a shared genealogy.
The best translation for "tribe" depends on the specific situation. If you're talking about a large, recognized tribal group, iwi is the most accurate.