千万, 最好, 尽量 — warnings and advice

规则 千万 = whatever you do — a WARNING, and nearly always with a negative:
千万别忘了。 千万不要迟到。 A positive one is rare: 千万记住。
最好 = you had better — mild advice: 你最好早点儿出发。
尽量 = as far as possible: 我们尽量在今天完成。
一定 = definitely — insistence or certainty: 明天你一定要来。