To one of my biggest assets, I wish you a very joyous birthday today!
I hope you know how much I value you not only as an employee but also as a friend and human being.
You lift the spirits of those around you, creating the very best atmosphere possible. The workplace would be seriously lacking without your bubbly personality brightening it up, day after day.
Thank you for being such a fantastic part of this company, and also for being such a fantastic person!
A very happy birthday to you!