I regularly spend my time mentally writing my resignation letter and every version has a sting in more than the tail. However when I eventually get to do it for real I will take ryanboy's advice. You just never know!
I'd go with either McNewPants letter or if you want to put something in the letter about why you are leaving what about/make it more personal how about:
It is with mixed feelings I am writing to you to give formal notice of my resignation from [insert job position]. As my contract states X months notice, my last working day will be Y. or As my contract states X months notice, my last working day will be Y, however I am currently owed Z days holiday, so would like my last day in the office to be ZZZ.
I have enjoyed my time working with you at ABC company, I have learned a lot from my colleagues here and am very grateful for the opportunities you have given me. However, I will be leaving for a new role which will offer me a new challenge and more scope to progress my career.
I wish you and the team well and am happy to discuss how to hand over my role to ensure a smooth as possible transition for the rest of the team.