These aren't going to be in a particular order, one that I am most thankful for over another, but Day 1 is the thing I am most grateful for and deserves my greatest and unending expression of thankfulness.
It is based off of this verse:
So then, just as you received Christ Jesus as Lord, continue to live your lives in him, rooted and built up in him, strengthened in the faith as you were taught, and overflowing with thankfulness. Colossians 2:6-7
#1- I am most thankful that I have received Christ Jesus as Lord and that I have a foundation that is rooted and continues to grow, am able to be strengthened and therefore can and should live a life that overflows with thankfulness.