The best work does not feel like work. You do not feel tired, heavy, bored- it feels like you are doing nothing extraordinary, it feels like you are playing! As LP Jack says-
The master in the art of living makes little distinction between his work and his play, his labor and his leisure, his mind and his body, his education and his recreation, his love and his religion. He hardly knows which is which. He simply pursues his vision of excellence at whatever he does, leaving others to decide whether he is working or playing. To him he is always doing both.
To find out what you are good at, see how you spend your time and then check to see if you can make your living from it. I love reading and writing- so I found that I was getting free books to read and getting paid to write!
What is your best work?