In regards to my absolute FAVORITE part...it's definitely putting it all together to see the final product.
I love doing pixel art for my assets, as I'm an artist. I love making the characters/drawings I need for my game, but to be honest, that is not always the most accomplishing aspect.
For me, I always tend to get the most satisfaction once I finish some kind of coding, finish an enemy, finish a map with all the events, utilizing a plugin I figured out...etc. I always get immense satisfaction when I look at the final product, and go, "I think all of that works!"