What do you think is the greatest way to say thank you?

anyway will do!! but if you really, really want something special..hhmmmmmm.. why don't you try surprising him?? ask him for a date, a very unique date perhaps! or make him a letter with your personal touch! or simply let him feel everyday of his life that he is very much appreciated thru simple gestures like being sweet to him.. giving a piece of candy or chocolates maybe.. write a poem, song, or any composition dedicated to him.. i used to do those stuffs to my best friends and to those people really close to me. and mind you, it works! try to express in the most eccentric and queer way you can! TRY TO BE DIFFERENT!

it depends upon how much u r obliged by that person then way of than comes out in different ways.