Complimentary close

Regards,
Best,
Peace,
Best regards,
Kind regards,
Thanks / Thank you (if you need to thank your clients for anything)
Thanks & regards
Best wishes,
When it comes to ending a letter you can use the following:
Yours truly,
Yours sincerely,
Sincerely yours,
Cordially,
Respectfully,
Respectfully yours,
Yours faithfully, (I don't think this one is very popular)