Depends on how you view life, in my opinion. Three years ago, probably as recent as two... I would have given a list that had quite a variety ranging from friends to love to my basic needs being met to an awesome job that paid well, yada yada yada. Mostly the list would consist of the things that other people had trained me to think would bring happiness. At each point in our life it's going to be different and it's going to change with the stages.
For now I start each day acknowledging that as a human, I barely deserve the air that I breath and that for every gift I receive I need to give that and more back. That every choice I make has a consequence (either positive or negative) and while I may not understand the correlation it will probably all play out as I learn the lessons life has to offer me.
Gratefulness and not focusing on the negative really does a good job at making me happy.