Boss, you have been here for  years says it all. You are an inspiration to others. Thank you for being an amazing guide and mentor. Wish you a happy work anniversary!
Dear [name of Boss],
I would like to congratulate you on completing  years at the company. The success you have achieved throughout these years is nothing but an inspiration for all of us. Your creative innovation for this organization has made this a great place to work, and your contribution to this organization has been extensive. Your hard work and dedication to your work is excellent. I would like to thank you for making a difference in our lives.
You motivate your employees. You take tough decisions in a very quick way. You always keep on supporting your colleagues in every way which is really a true inspiration for all of us. The many hours we have spent working on campaigns and projects together gave me the skills and great knowledge to take on this challenge, and I promise to give my 100% with full dedication to maintain the confidence and trust you have shown in me. Even words can neither qualify nor express how helpful your guidance and advice has been. Thank you for being there every time I needed you. I am forever grateful for your support! May you continue to be our boss for more years to come.
Your down-to-earth attitude makes you what you are, a winner. Continue to climb new heights with your hard work, passion and commitment. I wish you a happy work anniversary, Sir!