Having a ‘tone in your voice’  that's thoughtful and deliberate, you can say:   * I would like to express my gratitude to (NAME), for all the hard work you have done on this project.  Thank you. * Words cannot express how much you mean to me. * I am more grateful to you than you'll ever know. * I'm eternally grateful... * You have my deepest thanks. * I'll never forget your support and kindness.