I must be off now.

When you actually (в данный момент) leave you may say:

Good-bye. - formal, semi-formal

Good-bye for now.

Bye (then).

Bye now.semi-formal, informal

Bye-bye.

See you-very colloquial (разговорный).

See you later/tonight/tomorrow/on Saturday/next week, etc.

at the theatre/concert/party, etc.

(I'll) be seeing you.This means simply some time in the future. The phrases "All the best" and "Good luck" are used only in certain (определенных) circumstances. "All the best" is used when someone is seeing his friend off (to see off - провожать) for a long time, for example, or when leaving someone who is going for an interview (на собеседование) (for a job, etc), or in some similar (подобных) situations. "Good luck" may also be used in the latter (предыдущем) case (случае) or when someone is going to take an examination.

Good night. -when leaving someone late in the evening or before going to bed.

When leaving someone sometimes we want to be remembered to (передать привет) some member(s) of his family or a mutual (close)(близкий,закадычный) friend or acquaintance. The following phrases are used:

(Please) give my (kind) regards to ...-formal

egGive my kind regards to Professor Grant.

(Please) remember me to ... -formal, semi-formal

egPlease remember me to your parents.

Give my love to ... -informal, for close friends and relations.

egGive my love to Bill.

The person addressed usually (обычно) answers as follows (следующим образом):

Thanks/Thank you. I (certainly) will.