应该 (yīnggāi) is the standard, neutral word for 'should, ought to' — it expresses advice, expectation, or mild obligation. It's less forceful than 得 (děi 'must, have to'), which implies external necessity, and more common in everyday speech than the formal 应当 (yīngdāng). 该 (gāi) is a shorter, colloquial synonym used the same way.